Important Questions for CBSE Class 9 Science Chapter 13 – Why Do We Fall ill

Why do we fall ill Class 9 Important Questions for Class 9 Science Chapter 13

Science is a practical and experimental subject. One cannot excel in it by doing theory alone. Therefore, a detailed and well-structured preparation with the right approach is required to score well.

The chapter ‘Why do we fall ill’ discusses the reasons for falling ill and how one can prevent it. Also, one gets to know what precautions one should follow to be healthy and fit. The vital topics included in Chapter 13 Class 9 Science Important Questions are:

  • Why Do We Fall ill?  
  • Health and its Failure
  • Disease and Its Causes
  • Infectious Diseases

All the precautions, measures and remedies for being healthy are listed in the Important Questions Class 9 Science Chapter 13 for students to be health conscious. After referring to it, they will also learn various ways to maintain a healthy lifestyle.

Extramarks provide complete learning solutions so that students don’t lack behind in any aspect of their preparation. You can find the NCERT textbook, NCERT solutions, NCERT Exemplar, NCERT revision notes, NCERT formulas, NCERT-based mock tests and the Important Questions Class 9 Science Chapter 13 on their official website.

Get Access to CBSE Class 9 Science Important Questions 2022-23 with Solutions

Also, get access to CBSE Class 9 Science Important Questions for other chapters too:

CBSE Class 9 Science Important Questions

Sr No Chapters Chapter Name
1 Chapter 1 Matter in Our Surroundings
2 Chapter 2 Is Matter Around Us Pure
3 Chapter 3 Atoms and Molecules
4 Chapter 4 Structure of Atom
5 Chapter 5 The Fundamental Unit of Life
6 Chapter 6 Tissues
7 Chapter 7 Diversity in Living Organisms
8 Chapter 8 Motion
9 Chapter 9 Force and Laws of Motion
10 Chapter 10 Gravitation
11 Chapter 11 Work and Energy
12 Chapter 12 Sound
13 Chapter 13 Why Do We Fall ill
14 Chapter 14 Natural Resources
15 Chapter 15 Improvement in Food Resources

Class 9 Why Do We Fall Ill Important Questions with Solutions

The following important questions and their solutions are included in the Science Class 9 Chapter 13 important questions:

Question 1. Which one of the following is not a viral disease?

(a) Influenza

(b) AIDS

(c) Typhoid

(d)  Dengue

Answer 1. 

The answer is (c) Typhoid


Typhoid is caused by the bacterium called Salmonella typhi

Question 2. Which one of the following is not a bacterial disease?

(a) Anthrax

(b) Tuberculosis

(c) Cholera 

(d) Influenza

Answer 2. 

The answer is (d) Influenza


  • Cholera is caused by Vibrio cholera
  • Tuberculosis is caused by Mycobacterium tuberculosis
  • Anthrax is caused by Bacillus anthracis.
  • The Influenza virus causes influenza 

So, option d) is the correct answer.

Question 3. Which one of the following diseases is not transmitted by a mosquito?

(a) Brain fever

(b) Malaria

(c) Typhoid

(d) Dengue

Answer 3. 

The answer is (c) Typhoid


Typhoid is caused by contaminated food and water.

Question 4. Which one of the following diseases is not caused by bacteria?

(a) Tuberculosis 

(b) Anthrax

(c) Typhoid

(d) Malaria

Answer 4. 

The answer is (d) Malaria


Malaria is transmitted through a protozoan called Plasmodium vivax  

So, option d) is the correct answer

Question 5. Which one of the following diseases is caused by protozoans?

(a) Malaria

(b) Cholera 

(c) AIDS

(d) Influenza

Answer 5. 

The answer is (a) Malaria


Malaria is transmitted through protozoan Plasmodium vivax.

Question 6. Which of the following has a long-term effect on an individual’s health?

(a) Common cold

(b) Chickenpox

(c) Chewing tobacco

(d) Stress

Answer 6. 

The answer is (c) Chewing tobacco


Common cold and chickenpox are acute diseases that will not produce any long-term effects. Stress can be easily managed and would not have a long-term effect. Chewing tobacco is dangerous because it has a long-term impact.

Question 7. Which of the following could make you ill if you contact an infected person?

(a) High blood pressure

(b) Genetic abnormalities

(c) Sneezing

(d) Blood cancer

Answer 7. 

The answer is (c) Sneezing


Sneezing is the symptom of common and other respiratory diseases transmitted through contaminated air inhaling. Other options included in the question are not infectious diseases, so they are not transmitted through human contact.

Question 8. AIDS can’t be transmitted by

(a) sexual contact

(b) hugs

(c) breastfeeding

(d) blood transfusion

Answer 8. 

The answer is (b) hugs


AIDS is transmitted through the exchange of body fluids. If we hug each other, there cannot be an exchange of any body fluid, so hugs cannot transmit AIDS.

Question 9. Making antiviral drugs is more complex than making antibacterial medicines because

(a) viruses have a protein coat

(b) viruses are on the borderline between living and non-living

(c) a virus has very few biochemical mechanisms of their own

(d) viruses make use of host machinery 

Answer 9. 

The answer is (c) virus has very few biochemical mechanisms of their own


Antibacterial drugs stop a specific biochemical reaction in the bacteria, which kills the bacteria. However, in viruses, a few biochemical processes are difficult to target, thus making antiviral drugs difficult.

Question 10. Which one of the following causes kala-azar?

(a) Ascaris

(b) Trypanosoma

(c) Leishmania

(d) Bacteria

Answer 10. 

The answer is (c) Leishmania.

Question 11. If you live in an overcrowded as well as poorly ventilated house, you may suffer from which of the following diseases?

(a) Cholera 

(b) AIDS

(c) Airborne diseases

(d) Cancer

Answer 11. 

The answer is (c) Airborne diseases.

Question 12. Which disease is not transmitted by mosquitoes?

(a) Dengue

(b) Malaria

(c) Brain fever or encephalitis

(d) Pneumonia

Answer 12. 

The answer is (d) Pneumonia


Pneumonia is transmitted through the air and not mosquitos.

Question 13. Which one of the following is not important for individual health?

(a) Social equality and harmony

(b) Good economic conditions

(c) Living in a clean space 

(d) Living in a large and well-furnished house

Answer 13. 

The answer is (d) Living in a large and well-furnished house

Question 14. Choose the wrong statement

(a) Peptic ulcers are caused by eating acidic food 

(b) Genetic abnormalities can cause cancers

(c) High blood pressure is caused by excessive weight and lack of exercise.

(d) Staphylococci do not cause acne

Answer 14. 

The answer is (d) Staphylococci do not cause acne


Answer (d) is wrong as acne is a bacterial skin infection that is caused by staphylococci.

Question 15. We must not allow mosquitoes to breed in our surroundings because they

(a) are not important insects

(b) are vectors for many diseases

(c) bite and cause skin diseases

(d) multiply very fast and cause pollution 

Answer 15. 

The answer is (b) are vectors for many diseases


Mosquitos are dangerous because they spread diseases like Malaria, Dengue, Japanese encephalitis etc.

Question 16. You might be aware of the Polio Eradication Programme in your city. In these programmes, children are vaccinated because

(a) vaccination kills polio-causing microorganisms

(b) prevents the entry of the polio-causing organism

(c) it creates immunity in the body

(d) all the above

Answer 16. 

The answer is (c) it creates immunity in the body


Vaccines create immunity against the required pathogens against which it is designed.

Question 17. Viruses, which cause hepatitis, are transmitted through

(a) personal contact 

(b) water

(c) food

(d) air

Answer 17. 

The answer is (b) water

Question 18. Vectors could be defined as

(a) animals/insects carrying the infecting agents from a sick person to another healthy person

(b) microorganisms which cause many diseases

(c) infected person

(d) diseased plants

Answer 18. 

The answer is (a) animals/insects carrying the infecting agents from a sick person to another healthy person.

Question 20. ‘Penicillin’, a lifesaving antibiotic, was discovered by

  • Alexander Fleming 
  • Edward Jenner
  • H.G. Khorana
  • William Harvey.

Answer 20. a. Alexander Fleming

Question 22. Which one of the following deficiency diseases?

  • Diabetes
  • Malaria
  • Goitre
  • Tetanus

Answer 22. c. Goitre

Excessive blood sugar in the body causes diabetes. Malaria disease is caused by Plasmodium which is a protozoan parasite. Tetanus is an infection caused by toxin-producing bacteria. So these three are not classified as deficiency diseases. Goitre is caused due to iodine deficiency, so goitre is the correct answer.

Question 23. The chemical that kills or stops the growth of certain kinds of microbes is called-

  • Vaccines
  • Microbes
  • Antibiotics
  • fungi

Answer 23. c. antibiotics

Question 25. Rabies occurs through the bite.

  • Female Anopheles Mosquito
  • Female Culex mosquito
  • Rabid dog
  • housefly.

Answer 25. c. Rabid dog

Question 26. AIDS is caused by –

  • Bacteria
  • Virus
  • Protozoa
  • Fungi

Answer 26. b. Virus

Question 27. Identify a protozoan disease –

  • Ringworm
  • Measles
  • Diphtheria
  • Amoebiasis

Answer 27. d. Amoebiasis

Question 28. If you live in an overcrowded and poorly ventilated house, you may suffer from one of the following diseases.

  • Cancer
  • AIDS
  • Air borne diseases
  • Cholera.

Answer 28. c. airborne diseases

Question  29. Give two examples for each of the following

(a) Acute diseases

(b) Chronic diseases

(c) Infectious diseases

(d) Non-infectious diseases

Answer 29. 

(a) Acute diseases – Flu, Viral fever

(b) Chronic diseases – Tuberculosis, Elephantiasis

(c) Infectious diseases – Smallpox, chicken pox

(d) Non-infectious diseases- Diabetes, Cancer

Question 30. Name two diseases caused by Protozoans. What causes them?

Answer 30. 

Malaria is caused by Plasmodium vivax.

Leishmania causes Kala-azar.

Question 31. Which bacterium causes peptic ulcers? Who discovered this bacteria for the first time?

Answer 31. 

Peptic ulcers are transmitted through the bacterium Helicobacter pylori. Barry J. Marshall and Robin Warren discovered this bacteria.

Question 32. What is an antibiotic? Give two examples

Answer 32. 

Antibiotics are the chemical agents which kill bacteria or fungi by blocking their specific biochemical pathways. Ex: Streptomycin, Azithromycin.

Question 33. Fill in the blanks

(a) Pneumonia is an example of ——— disease.

(b) Many skin diseases are caused by———.

(c) Antibiotics commonly block biochemical pathways important for the growth of ———.

(d) Living organisms carrying the infecting agents from one person to another are called ———.

Answer 33. 

(a) Pneumonia is an example of a bacterial disease.

(b) Fungi cause many skin diseases.

(c) Antibiotics commonly block biochemical pathways important for the growth of bacteria.

(d) Living organisms carrying the infecting agents from one person to another are called vectors.

Question 34. Name the target organs for the following diseases

(a) Hepatitis targets———.

(b) Fits or unconsciousness targets ———.

(c) Pneumonia targets ———.

(d) Fungal disease targets ———.

Answer 34. 

(a) Hepatitis targets the liver.

(b) Fits or unconsciousness targets the brain.

(c) Pneumonia targets the lungs.

(d) Fungal disease targets the skin.

Question 35. Who discovered ‘vaccine’ for the first time? Name two diseases which can be prevented by using vaccines.

Answer 35. 

Edward Jenner discovered a vaccine for the first time. Polio and tetanus are diseases that can be prevented by using vaccines.

Question 36. Fill in the blanks 

 (a) ______ disease lasts for many days and causes______ in the body. 

 (b)_______ disease lasts for a few days and does not cause any long-term effects on the body. 

(c) _______ is defined as the physical, mental as well as social well-being of a person  

 (d) common cold is ______sickness.

(e) Many skin diseases are caused by______.

Answer 36. 

(a) A chronic disease lasts for many days and causes long-term effects on the body. 

 (b) Acute disease lasts for a few days and does not cause any long-term effects on the body. 

(c) Health is defined as the physical, mental as well as social well-being of a person.  

 (d) the common cold is a viral sickness.

(e) Many skin diseases are caused by Fungi.

Question 37. Classify the following diseases as infectious or non-infectious.

(a) AIDS

(b) Tuberculosis

(c) Cholera

(d) High blood pressure

(e) Heart disease

(f) Pneumonia

(g) Cancer

Answer 37. 

(a) AIDS – Infectious disease

(b) Tuberculosis – Infectious disease

(c) Cholera – Infectious disease

(d) High blood pressure – non-Infectious disease

(e) Heart disease non – Infectious disease

(f) Pneumonia – Infectious disease

(g) Cancer – non-Infectious disease

Question 38. Name any two groups of microorganisms from which antibiotics could be extracted.

Answer 38. 

Bacteria and fungi

Question 39. Name any three diseases transmitted through vectors.

Answer 39. 

  • Malaria is transmitted through mosquitoes.
  • Rabies is transmitted through mad dogs.
  • Elephantiasis is transmitted through mosquitoes.

Question 40. Explain Rationale 

 (a) A balanced diet is necessary to maintain a healthy body. 

 (b) The health of an organism depends on the surrounding environmental conditions. 

 (c) Our environment must be free from standing water. 

 (d) Social harmony, as well as good economic conditions, are necessary for good health.

Answer 40. 

  1. a) Food is necessary for the growth and normal functioning of our body. A balanced diet is one that contains all the essential nutrients in the right amounts. A balanced diet provides a required amount of biomolecules like carbohydrates, lipids, proteins, vitamins and minerals that are essential for our growth and body functioning. An unhealthy diet can lead to many physical and physiological disorders.
  2. b) Health is necessary for physical, mental and social well-being and comfort. Health depends on our immediate surroundings; when our environment is unclean, we are likely to catch diseases. We get cold as well as cough in the rainy as well as winter seasons. Our environmental conditions are vital to remaining healthy.
  3. c) Stagnant water is the breeding ground for vectors like mosquitoes. Mosquitoes cause many diseases like Malaria, Dengue, Elephantiasis, chikungunya etc. To stop the spread of these particular diseases, our surroundings should be free of stagnant water.
  4. d) Human beings are social organisms. We live in different kinds of localities and environments. To lead a healthy life, we need to keep our environment clean. We have hygienic food to lead a healthy life which depends on our economic conditions. We often catch diseases; to get treatment, we need money, and our financial status should be sufficient to be able to afford the treatments.

Question 41. What is a disease? How many types of illnesses have you studied? Give examples.

Answer 41. 

Each organ in the organ system has specific functions to perform. When this function changes or is disrupted, it leads to disease. These changes cause some adverse effects on the body’s normal functioning; the observed differences are called the disease’s signs and symptoms. 

 types of diseases 

  • Acute: 

 Diseases that are short-lived are called acute diseases. Ex.: Cold 

  • Chronic: 

 Illnesses that last a long time, sometimes even for a whole lifetime, are called chronic diseases. Example: elephantiasis. 

  • Infectious Diseases: 

 Diseases caused by microorganisms are called infectious diseases. These diseases could be transmitted from one person to another through vectors or other means such as air, food, and water. Example: Typhoid 

  • Non-contagious: 

 Diseases are not caused by infectious agents. Its causes are different, but it is not external causes like microbes that can spread in the community. Instead, these are primarily internal, non-infectious causes. Example: high blood pressure. 

 Question 42. What do you mean by symptoms of illness? Explain this using two examples. 

 Answer 42. 

When our body encounters a disease, it causes the body to function abnormally. These abnormal functions are called symptoms. 

For example, a malaria patient would have symptoms such as high fever with tremors, chills, headache, nausea, vomiting, abdominal pain, diarrhoea, etc. 

A tuberculosis patient would have symptoms such as fever, chills, loss of appetite, blood in cough, chest pain, etc. 

Question 43. Why is this Immune system so important for our health?

Answer 43. 

The immune system is the spine of our body. The immune system consists of different types of cells that kill infectious microorganisms. When a pathogen enters the body, antibodies will be released in response to the pathogen. These antibodies kill pathogens and protect us from disease.

Question 44. What precautions should be taken to justify that “prevention is better than cure”? 

Answer 44. 

The following precautions are taken to prevent disease: 

  1. Care for personal and public hygiene. 
  2. Raising awareness of the causes, symptoms and causes of diseases
  3. Regular exercise to strengthen immunity 
  4. Vaccination 

Question 45. Why do some children get sick more often than others living in the same area? 

Answer 45. 

 Some of the reasons are listed below. 

  •  A lack of a balanced diet leads to malnutrition 
  •  Weak immune system 
  •  Living in unsanitary and cramped conditions 
  •  Lack of pure drinking water and unavailability of hygienic food 
  •  Lack of vaccinations 

Question 46. Why don’t antibiotics work for viral diseases? 

 Answer 46. 

 Antibiotics act on bacteria as well as fungi by blocking certain biochemical activities that are necessary for bacterial life. On the other hand, the virus uses the host’s machinery to survive and replicate. Therefore, antiviral drugs work differently than antibiotics.

They interfere with viral enzymes to destroy them. 

Question 47. Being exposed to or infected with an infectious microbe does not necessarily mean developing an overt disease. Kindly explain. 

Answer 47. 

Our immune system fights and eliminates pathogens by killing them to protect us from disease. When exposed to a specific pathogen, our immune system sends out active cells that kill and eliminate the pathogen. Because of our immune system, we often don’t get the disease despite being exposed to the infectious microbe.

Question 50. How does a physician diagnose diseases?

Answer 50. When a person is infected with a disease, it may cause malfunctioning of organs or body parts, producing external symptoms of diseases. Based on the symptoms and physical examination of the person, the disease is diagnosed by the physician.

Question 52. Identify commonly used approaches to treating infectious diseases.

Answer 52. Two ways can treat infectious diseases:

  1. By reducing its effect by using medications that may reduce their symptoms.
  2. By killing the bacteria causing it using antibiotics.

Question 53. Name four factors that are necessary to be a  healthy person. 

 Answer 53. 

  • Hygienic Environment: 

 Clean water and a Hygienic environment can keep us away from waterborne and airborne diseases. 

  • Personal hygiene: 

 We must keep our bodies clean and hygienic to be healthy.

  • Food: 

 Hygienic and nutritious foods stimulate the immune system; Therefore, a balanced diet is essential to be healthy. 

  • Vaccination: 

 Vaccination against serious diseases is essential to protect ourselves from them.

 Question 54. Why is AIDS considered a “syndrome” and not a disease? 

 Answer 54. 

HIV AIDS is caused by the HI virus, which enters our bodies through bodily fluids such as blood and breast milk or through sexual means. HIV spreads to the lymph nodes and blocks our immune system. This creates a state where our body cannot fight off even minor illnesses like a cold. 

 A person with HIV could get pneumonia from a common cold; A mild intestinal infection can develop into severe diarrhoea. With HIV, a person has no specific symptoms. Instead, he will have a variety of diseases. Therefore, HIV is called a syndrome and not a disease. 


Question 55. List two essential conditions for good health. 

Answer 55. 

Two essential requirements for good health are: 

  1. State of physical, mental, as well as social well-being. 
  2. Clean and hygienic Environment. 

Question 56. Name two essential conditions for being disease-free. 

Answer 56. 

 Two essential conditions for being free from the disease are: 

  1. Personal and domestic hygiene. 
  2. Clean environment 

Question 57. Are the answers to the previous questions necessarily the same or different? Why?

Answer 57. 

The answers to the above questions will vary because a person can be disease free but not mentally, socially and financially healthy.

Question 58. List three reasons why you would feel that you are sick and should see a doctor. Would you still go to the doctor if only one of these symptoms were present? Why or why not?

Answer 58. 

  1. Headache 
  2. Cough 
  3. Loose motions 

These indicate that a disease may be present but do not predict what the disease is. Hence, one would still visit the doctor to treat the above symptoms and find the cause.  

 A doctor should be consulted for proper treatment even if only one symptom is present. 

Question 59. In which of the following situations do you think the long-term effects on your health are more severe? Give reasons.

  •  for jaundice, 
  •  for lice, 
  •  for acne

 Answer 59. Lice and acne have no long-term effects on the human body. However, the consequences of jaundice are long-term and could be severe. 

 Jaundice is a chronic disease that affects the whole body and takes longer to heal fully. 

Question 60. Why are we generally advised to eat soft, nutritious foods when we are sick? 

Answer 60. During illness, the body becomes weak, and the digestive system does not function properly. Therefore, eating easily digestible foods during this period is necessary, and nutrient-rich foods are recommended. The body’s immunity decreases during illness or infection. Therefore, liquid or semi-liquid and nutritious food is given for speedy recovery. 

Question 61. What are the different ways by which infectious diseases are spread? 

 Answer 61. Infectious diseases are generally transmitted through the following ways: water, air, vectors such as mosquitoes, sexual contact, physical contact with the infected person, or the use of the infected person’s clothing, bedding, utensils, etc. 

Question 62. What precautions can you take at your school to reduce the incidence of infectious diseases? 

 Answer 62. Some of the precautions we can take in our school to reduce the cases of infectious diseases are: 

  1. Drink clean and hygienic water. 
  2. Avoid the accumulation of water in your surroundings. 
  3. Keep the bathroom clean and tidy.
  4. Avoid eating uncovered foods and other edibles. 
  5. Bath daily 
  6. Eat a balanced diet.
  7. Keep the environment clean to prevent mosquitoes from breeding. This prevents the spread of vector-borne diseases. 

Question 63. What is immunisation?

Answer 63. The method of stimulating our immune system using vaccines that help the body to fight infectious diseases is called immunisation. 

Question 64. What are the immunisation programs at your nearest health centre? Which of these diseases are the biggest health problems in your area? 

Answer 64. 

The following immunisation program is available at the nearest health centre in our city 

  1. Vaccination for infants: DPT, BCG, Polio, Measles and MMR. 
  2. For Children – Typhoid, TT, DT, Smallpox and TAB.
  3. For a pregnant woman: TT and hepatitis B. 

Diseases such as typhoid, poliomyelitis and measles are the major health problems in our area.

Question 65. How often have you been ill in the last year? What diseases were there? 

(a). Think of a change you could make in your habits to avoid some/most of the above diseases.

(b). Think of a change you would like to see in your environment to avoid one or most of the above diseases. 

 Answer 65. 

I got sick twice in the last year. First, I suffered from diarrhoea and then dengue

(a) The changes I have made in my habits after suffering from these diseases are: 

  (i) I drink clean water from a water purifier and I frequently wash my hands with soap to keep them germ-free. 

  (ii) I keep my environment very clean to ensure that the disease carriers do not multiply in my surroundings.

(b) One change I would like to see in our environment in order to have a healthy society is to provide people with clean drinking water. Consumption of contaminated water is the main cause of many infectious diseases.

Question 66. A doctor/nurse/health worker is exposed to more sick people than others in the community. Find out how he/she keeps himself/herself from getting sick. 

 Answer 66. 

Some important precautions that the physician/nurse/health worker must take when treating people who are more ill than others in the community are – 

(a). Don’t forget to wear a mask if you come into contact with an infected person. 

(b). Drink purified water. 

(c). Do not neglect cleanliness and personal hygiene.

 (d). Protect yourself adequately when moving into an infected region

(e). Eat nutritious foods and follow a healthy diet. 

Question 67 Conduct a survey in your neighbourhood to identify the three most common diseases. Suggest three steps local authorities could take to reduce the incidence of these diseases. 

 Answer 67. 

The following are observed as the three most common diseases in any neighbourhood: Cold and cough, dizziness, and Malaria. 

Some of the below preventive measures can be taken to reduce their occurrence: 

 (a). Drink fresh, uncontaminated and clean water.

(b). maintaining sanitary hygiene. 

(c). Spread awareness about various preventive measures with the help of posters and brochures. 

Question 68. A baby cannot tell its caregivers that it is sick. What would help us to know 

(a) that the baby is sick? 

(b) What is the illness? 

Answer 68. 

(a). This can be determined by observing behavioural changes in the child, such as


  • Inadequate food intake 
  •  Constant crying 
  •  Frequent mood swings 

 (b). The disease can be determined based on the symptoms or signs that the child is exhibiting. Symptoms can include loose movements, vomiting, pale skin and fever.

Question 69. Under what subsequent conditions, could a person possibly get sick? 

(a) in recovery from Malaria. 

(b) if you have recovered from Malaria and are taking care of someone with chickenpox. 

(c) when fasting for four days after recovering from Malaria and looking after someone suffering from chickenpox. Why? 

Answer 69. 

A person is more likely to get sick if he fasts for four days after recovering from Malaria and looks after someone who is suffering from chickenpox. This is because if someone fasts during the recovering period, their immune system becomes weaker since it doesn’t get the required nutrition to recover. Due to this, the immune system cannot fight the chickenpox virus transmitted by the infected person. And hence, they can catch the disease. 

Question 70. Under which of the following conditions are you most likely to get sick? 

(a) when taking tests.

(b) if you have travelled by bus and train for two days. 

(c) when you come in contact with your friend who has measles. Give reasons.

 Answer 70. 

A person is more likely to get sick if they come into contact with their friend who has measles. Because measles is highly contagious and could spread quickly through the air, by coughing and sneezing. Therefore, if a friend has measles, it is recommended to stay away from him or her to avoid catching an infection.

Question 71. Why are we often advised to eat semi-liquid or liquid, nutritious foods when we are ill? 

Answer 71. if we are sick, our body becomes fragile, and our digestion also becomes bad. Therefore, we are advised to eat soft and nutritious food when suffering from disease, as we need complete nutrition for faster recovery. 

Question 72. What are the different ways by which infectious diseases are spread? 

Answer 72. Infectious diseases can be spread through the following 

  1. Through contaminated food and water 
  2.  Through the atmosphere 
  3.  Through vectors or vectors (flies, mosquitoes, etc.) 
  4.  Through direct skin contact or sexual contact 
  5.  Through cuts or wounds 

Question 73 What precautions can you take in your school to reduce the incidence of infectious diseases? 

Answer 73. In my school, to reduce the incidence of infectious diseases, I would take the following precautions: 

  1. Keep distance from an infected person 
  2.  Keep surroundings clean 
  3.  Wash hands regularly.
  4.  Would not eat open/uncovered food.

Question 74. What is vaccination? 

Answer 74. Vaccination is a process in which the weak or dead microbes of a specific disease are injected into the body so that they grow memory cells (GB) but do not cause the disease. For example, vaccination against polio, tetanus etc. A vaccinated person will develop resistance to the specific disease.

Question 75. Why are antibiotics not effective for viral diseases?

Answer 75. Viruses are different from bacteria. They do not have cell walls which antibodies can attack. Thus, antibodies are ineffective against viral diseases. Antibiotics tend to break the bacterial cell wall as well as block the bacterial pathways. So, they are effective on bacterial infections only.

Question 76. How can the disease be prevented?

Answer 76. The diseases can be prevented by taking the following measures given below:

  1. Avoid crowded places
  2.  Maintain public hygiene 
  3.  Wash hands regularly 
  4.  Eat a balanced diet and maintain personal hygiene.
  5.   Increase immunisation. 

Question 77. Give two examples of each of the following: 

  •  Chronic diseases
  • Infectious Diseases
  • Non-infectious Diseases.

Answer 77. 

  • Chronic diseases: Chronic diseases – Diabetes, arthritis
  • Infectious Diseases: Infectious diseases – Chickenpox, Influenza
  • Non-infectious Diseases: Non-infectious diseases – asthma, obesity

Question 78. Give the difference between vaccines and antibiotics.

Answer 78. Differences between vaccines and antibiotics are given below:

Vaccines Antibiotics
Vaccines are defined as the suspension of weak or dead microbial agents of the same disease in the person so that he develops an immune against that disease. Antibiotics are medicines made by living microbes and can be used for one or more diseases.
They are given before the person is infected to protect him. They are given after the person is infected with the disease.
They provide immunity or protection against disease for a more extended period. Antibiotics fight the disease by attacking the bacteria causing it.  

Question 79. A patient’s body has lost its power to fight infection. What disease could the patient suffer from? Name the pathogen and describe any two routes of transmission from the patient to another person.

Answer 79. The patient is suffering from the disease AIDS, which is Acquired Immune Deficiency Syndrome. 

AIDS is caused by the HIV virus– Human Immunodeficiency virus.

Two modes of transmission are

  1.  By sexual contact with the infected person.
  2.  By the use of the infected syringe or needle.

Question 80. What are the conditions favouring air-bone diseases?

Answer 80. Conditions that favour airborne infections are:

  1.  Close contact with the infected person.
  2.   Overcrowded areas 
  3.   Poor – Ventilation

Question 81. What are the various dimensions of health?

Answer 81. The different dimensions of health are as follows:

  1. Physical Dimensions- It states the perfect functioning of all the organs and parts of the body.
  2. Mental Dimensions – The state of mental balance, harmony and peace between any individual and their environment.
  3. Social Dimensions – the person is socially sound when he shares a good bond with his surroundings and the people who are around him; that is when he has a good job, a good house, a happy family, and helpful friends.

Question 82. What are the causes of diseases?

Answer 82. The various causes of diseases are:

  1.  Microorganisms – Bacteria, viruses, fungi, protozoa and worms are causing the most communicable diseases.
  2.  Malfunctioning in the parts of the body or the body organs.
  3. Nutritional deficiency.
  4. Genetic factors.
  5.  Polluted environment.

Question 83. Give the schedule of immunisation for infants as well as children up to six years.

Answer 83. The schedule for the immunisation for infants and children up to six years is as follows:

Age Vaccine No. of doses


3-9 months DPT 3 doses at intervals of 1-2 months
Polio 3 doses at intervals of 1-2 months
9-12 months Measles 1
18-24 months DPT 1 (booster)
Polio 1 (booster)
Children 5-6 years Typhoid 2

Question 84. Explain giving reasons –

  • A balanced diet is necessary to maintain a healthy body.

Answer 84. A balanced diet contains all the nutrients needed for maintaining proper health. It is also required for good growth and repair of the damaged cells.

  • The health of an organism depends upon surrounding environmental conditions.

Answer 84. The surrounding environmental conditions play a vital role in the maintenance of health. For example, we feel uneasy when–

  1. Our surroundings are dirty, the garbage is not collected or disposed of from time to time.
  2. Drains are not cleaned, and water contamination is in the streets or open spaces. 

Question 85. What is immunisation, immune system, and immunity?

Answer 85. Immunisation is the method by which any immunised person develops certain resistance to a specific disease. The immune system is the system that protects the body against pathogens which can lead to infections. Immunity refers to our body’s capacity to resist and overcome diseases.

Question 86. Name the agents and the diseases they cause. ?

Answer 86. A list of the agents and the disease caused by them is as follows;

Agents Disease Caused
Bacteria Tuberculosis, whooping cough
Viruses AIDS, chicken pox, Common cold
Fungi Skin infection, ringworm, athlete’s foot
Protozoa Sleeping sickness, Malaria, amoebic dysentery
Worms Giardiasis, Amebiasis

Benefits of Solving Important Questions Class 9 Science Chapter 13

Science subjects require that students regularly revise and practise a lot of questions before the final exams. Extramarks team has collated important questions from various credible sources and compiled step-by-step instructions-based solutions for each of the questions. 

Below are a few benefits for students to refer to Extramarks important questions Class 9 Science Chapter 13:

  • Class 9 Science Chapter 13’s Important Questions range from easy to challenging.
  • The solutions are provided after detailed research and analysis of the requirements of CBSE past years’ papers.
  • The questions cover all the important concepts and topics from the chapter.
  • The Important Questions Class 9 Science Chapter 13 are updated per the CBSE latest curriculum, ensuring that they are up to the mark.

Q.1 Answer the following questions:
(a) What is the difference between mass by mass percentage and volume by volume percentage of a solution? Give their formulae.
(b) Calculate the mass by mass percentage of a solution containing 20 g of sodium chloride dissolved in 130 g of water.
(c) Is a true solution stable or unstable? Give reason to support your answer.



Mass by mass percentage Volume by volume percentage
1. Mass by mass percentage of a solution is the mass of solute (in gm) present in 100 gm of solution. 1. Volume by volume percentage is the volume of solute (in mL) present in 100 m of solution.
2. It is given as:

Mass  of  solute  (in  g)Mass  of  solution  (in  g)100
2. It is given as:

Volume  of  solute  (in  mL)Volume  of  solution  (in= mL)100

(b) Mass by mass percentage is given as

Mass of solute (in g)Mass of solution (in g)100

Given mass of sodium chloride = 20 g
Mass of water (solvent) = 130 g
Mass of solution = 20 + 130 = 150 g
Thus, the mass percentage of solution =


(c) True solutions are stable in nature as their particles do not settle down when left undisturbed.

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FAQs (Frequently Asked Questions)

1. How should I prepare for Class 10 board examinations from Class 9?

To prepare for Class 10 board examinations from Class 9, you must study all the subjects of Class 9 properly and ensure all your concepts are clear. Also, instil the habit of reviewing the syllabus regularly and revising from time to time to score well in your Class 10 board examinations.

2. Can I score full marks in Class 9 Science?

Yes, you can score full marks in Class 9 Science if you study the subject consistently, learn questions and answers given in the textbook thoroughly, take experts guidance, practice problems from different sources like NCERT textbook, NCERT solutions, NCERT Exemplar and the important questions Class 9 Science Chapter 13, revised from time to time and give regular mock tests.