NCERT Solutions for Class 12 Biology Chapter 8 Human Health and Disease


Why is that once a person starts taking alcohol or drugs, it is difficult to get rid of this habit? Discuss it with your teacher.


Drug and alcohol consumption has an addictive nature associated with a temporary feeling of well-being. With repeated use of drugs, the tolerance level of the drug receptors present in our body increases. Consequently, the receptors respond only to higher doses of drugs or alcohol leading to greater intake and addiction.


Name the primary and secondary lymphoid organs.


(a) Primary lymphoid organs include the bone marrow and the thymus.

[Explanation: In these primary lymphoid organs, immature lymphocytes differentiate into antigen-sensitive lymphocytes.]

(b) Secondary lymphoid organs are the ‘Peyer’s patches of small intestine’, spleen, lymph nodes, tonsils, and appendix.

[Explanation: The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.]


Discuss with your teacher what does ‘a suitable gene’ means, in the context of DNA vaccines.


A ‘suitable gene’ refers to a specific DNA segment which can be injected into the body of the host to produce specific proteins or antibodies. These antibodies can kill specific disease-causing organism in the host and provide immunity.

[Explanation: DNA introduced into the cells remains active for a long period and this therapy provides a long term protection from the disease.  This powerful strategy is also used to reestablish the normal equilibrium and overcome a diseased condition in host.]


What measures would you take to prevent water-borne diseases?


Water-borne diseases such as amoebiasis, ascariasis, cholera, typhoid, hepatitis B are spread by drinking contaminated water. These water-borne diseases can be prevented by consumption of clean drinking water, food, vegetables, fruits, etc. Public hygiene measures like proper disposal of waste and excreta, periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks need to be done.


In which way has the study of biology helped us to control infectious diseases?


Numerous advancements made in biological science have helped us to deal with many infectious diseases efficiently. Study of the biology of many disease-causing organisms has helped us to understand the lifecycle and disease-causing properties of the causative agent. This has helped in the production of new and safer vaccines. Through vaccines and immunisation programmes, many infectious diseases like polio, diphtheria, pneumonia and tetanus have been controlled to a large extent.  Discovery of antibiotics and various other drugs has also enabled us to effectively treat infectious diseases.


How is a cancerous cell different from a normal cell?


Normal cell

Cancerous cell

Normal cells show a property called contact inhibition by virtue of which contact with other cells inhibit their uncontrolled growth and they stop dividing.

Cancerous cells lack the property of contact inhibition. Therefore, they continue to divide, giving rise to masses of cells called tumors.


Normal cells undergo differentiation after attaining specific growth.

Cancerous cells do not undergo differentiation.

These cells remain confined at a particular location.

When the cancerous cells become malignant, they grow very rapidly, invading and damaging the surrounding normal tissues.


What is the mechanism by which the AIDS virus causes deficiency of immune system of the infected person?


AIDS is caused by the Human Immunodeficiency Virus (HIV), via sexual or blood-blood contact. After getting into the body of the person, the virus enters into macrophages where the RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into the host cell’s DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus and in this way, they act as an HIV factory. HIV enters into helper T-lymphocytes (TH), replicates and produces progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This repeated process leads to a progressive decrease in the number of helper T-lymphocytes in the body of the infected person, thereby decreasing the immunity of a person.


Draw a well-labelled diagram of an antibody molecule.


A well-labelled diagram of an antibody molecule is depicted below:


In your view what motivates youngsters to take to alcohol or drugs and how can this be avoided?


There are several factors responsible for motivating youngsters towards alcohol or drugs. Curiosity, need for adventure, excitement and experimentation constitute common causes, which motivate youngsters to try drugs and alcohol.  Television, movies, newspapers, the internet also help to promote the perception that drugs and alcohol can solve many big problems. Other factors that are associated with rampant drug and alcohol abuse among adolescents are unstable or unsupportive family structures and peer pressure.

Preventive measures against alcohol and drug abuse are as follows:

(i) Avoid undue peer pressure -  A child should not be pressurised unduly to perform beyond his/her threshold limits be it studies, sports or other activities.

(ii) Education and counselling - Educating and counselling him/ her to face problems and stresses, and to accept disappointments and failures as a part of life. Children should channelise their energy into healthy pursuits like sports, reading, music, yoga and other extracurricular activities.

(iii) Seeking help from parents and peers - Help from parents and peers should be sought immediately so that they can guide appropriately.

(iv) Looking for danger signs - Alert parents and teachers need to look for and identify the danger signs. Appropriate measures would then be required to diagnose the malady and the underlying causes.

(v) Seeking professional and medical help – Youngsters who have become drug/alcohol addicts should take help from highly qualified psychologists, psychiatrists, and attend de-addiction and rehabilitation programmes to help themselves.


Do you think that friends can influence one to take alcohol/drugs? If yes, how may one protect himself/herself from such an influence?


Yes, friends can influence one to take drugs and alcohol. A person can take the following steps to protect himself/herself against drug abuse:

(a) Increase your will power to say no to alcohol and drugs. One should not experiment with alcohol for adventure and excitement.

(b) Avoid the company of friends who take drugs and alcohol.

(c) Seek help from family and peers to get rid of the habit.

(d) Take proper counselling and professional help about drug and alcohol abuse.

(e) Utilise your energy in constructive extra-curricular activities.


List the harmful effects caused by alcohol/drug abuse.


Addiction to alcohol/drugs is a very serious problem the present-day society is facing and is a cause of real concern.

Effects of alcohol:

On an individual: Alcohol causes short-term as well as the long-term effect on an individual.

  1. Short-term effects include Slurred speech, drowsiness, vomiting, diarrhoea, headaches, breathing difficulties, unconsciousness etc.
  2. Long-term effects include High blood pressure, stroke, liver disease, nerve damage, vitamin B1 deficiency, ulcers, malnutrition etc. 

Also, it is advisable for pregnant women to avoid alcohol as it may retard the growth of the baby.

On the family: Long term consumption of alcohol by any family member can have negative effects on the family. It may lead to several domestic problems like quarrels, sexual assault, domestic violence, frustrations, insecurity, etc.

On the society: Increased on-the-job injuries and loss of productivity, rash behaviour, violence and loss of interest in social activities.

Similarly, an individual who is addicted to drugs creates problems for himself as well as for the family and society.


Effects of drugs:

On an individual: It weakens the immune system and causes cardiovascular diseases. Injected drugs can cause infections of the blood vessels and heart valves, dramatic fluctuations in appetite and increase in body temperature. Susceptibility of HIV infection is most common in these individuals as they share common needles while injecting drugs in their body.

On the family and society: A person addicted to drug creates a family as well as a social problem as he becomes frustrated, irritated, confused and may become anti-social.

The most common warning signs of drug and alcohol abuse among youth include drop in academic performance, unexplained absence from school/college, lack of interest in personal hygiene, withdrawal, isolation, depression, fatigue, aggressive and rebellious behaviour, deteriorating relationships with family and friends, loss of interest in hobbies, change in sleeping and eating habits, fluctuations in weight, appetite, etc.


Explain what is meant by metastasis.


The pathological process of spreading cancerous cells to the different parts of the body is called metastasis. The property of metastasis is exhibited by malignant tumours. The malignant tumours are a mass of proliferating cells called neoplastic or tumour cells. These cells grow very rapidly, invading and damaging the surrounding normal tissues. As these cells actively divide and grow, they also starve the normal cells by competing for vital nutrients. Cells sloughed from such tumours travel through blood and lymph, reach distant sites and rise to a new tumour.

[Explanation: Metastasis is a complex process and to successfully colonise a distant area in the body, the cancer cell must complete a series of steps to form a clinically detectable lesion. The most common sites of cancer metastasis are the bone, liver and lung ]. 

The steps of metastasis include:

Separation from the primary tumour.
Invasion through tissues around the initial lesion.
Entry into the blood vessels and lymphatic vessel.
Reaching the distant organ like lungs, liver, bone.
Formation of a new tumour along with new blood vessels feeding the tumour.


What are the various routes by which transmission of human immunodeficiency virus takes place?


AIDS is caused by the Human Immuno deficiency Virus (HIV), a member of a group of viruses called retrovirus, which has an envelope enclosing the RNA genome. Transmission of HIV-infection generally occurs in the following ways:

(a) Sexual contact with an infected person.

(b) Transfusion of contaminated blood and blood products.

(c) Sharing infected needles like in the case of using intravenous drug abusers.

(d) From an infected mother to her child through the placenta.


Differentiate the following and give examples of each:
(a) Innate and acquired immunity
(b) Active and passive immunity



Innate immunity

Acquired immunity

Innate immunity is a non-specific type of defence that is present at the time of birth.

Acquired immunity is pathogen-specific. It is characterised by memory.

It is inherited from parents.

It is acquired after the birth when the host encounters a pathogen.

It functions by providing various barriers such as

(i) Physical barriers (ii) Physiological barriers (iii) Cellular barriers and (iv) cytokine barriers against the entry of foreign infectious agents.

It functions by producing primary and secondary immune responses, which are mediated by B−lymphocytes and


It does not have a specific memory.

It is characterised by an immunological memory upon encounter with the antigen.




Active immunity

Passive immunity

It is a type of acquired immunity wherein the body produces its own antibodies against living or dead microbes or other proteins.

It is a type of acquired immunity where readymade antibodies are administered directly to protect the body against foreign agents.

It is slow and takes time in generating antibodies and giving responses.

It is fast and provides immediate relief.

For example: Injecting the microbes deliberately during immunisation or infectious organisms gaining access into the body during natural infection induces active immunity.

Transfer of antibodies from the mother’s milk to the infant through yellowish fluid colostrums during the initial days of lactation is an example of passive immunity.


The following are some well-known abbreviations, which have been used in this chapter. Expand each one to its full form:
(a) MALT
(b) CMI
(c) AIDS
(d) NACO
(e) HIV


(a) MALT- Mucosa-Associated Lymphoid Tissue

(b) CMI- Cell-Mediated Immunity

(c) AIDS- Acquired Immuno Deficiency Syndrome

(d) NACO- National AIDS Control Organisation

(e) HIV- Human Immuno Deficiency virus


How does the transmission of each of the following diseases take place?
(a) Amoebiasis
(b) Malaria
(c) Ascariasis
(d) Pneumonia


Sl. No





Mode of transmission



Entamoeba histolytica

It is a vector-borne disease.  The housefly is the vector which acts as a mechanical carrier and transmits the parasite.

The infection spreads through the drinking of contaminated water and food.




species of Plasmodium

It is a vector-borne disease that spreads by biting of infected female Anopheles mosquito.



Ascaris lumbricoides

A healthy person acquires this infection through contaminated water and food.



Streptococcus pneumoniae and Haemophilus influenzae

A healthy person acquires this infection by inhaling the droplets/aerosols released by an infected person or by sharing glasses and utensils with an infected person.


What are the various public health measures, which you would suggest as safeguard against infectious diseases?


Public health measures are very important for the prevention and control of various infectious diseases. These measures help to safeguard against infectious diseases. Some of these measures are:

a. Maintenance of personal and public hygiene: It is very important for the prevention and control of many infectious diseases. Personal hygiene includes keeping the body clean, consumption of clean drinking water, food, vegetables, fruits, etc. and public hygiene includes proper disposal of waste and excreta, periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks. These measures are particularly essential for water-borne diseases such as typhoid, amoebiasis and ascariasis.

b. Prevention of air-borne diseases: In cases of air-borne diseases such as pneumonia and the common cold, in addition to the above measures, close contact with the infected persons or their belongings should be avoided.

c. Prevention of vector-borne disease: For diseases such as malaria and filariasis that are transmitted through insect vectors, the most important measure is to control or eliminate the vectors and their breeding places. This can be achieved by

(i) avoiding stagnation of water in and around residential areas

(ii) regular cleaning of household coolers

(iii) use of mosquito nets

(iv) introducing fish like Gambusia in ponds that feed on mosquito larvae

(v) spraying of insecticides in ditches, drainage areas and swamps, etc.

(vi) doors and windows should have wire mesh to prevent the entry of mosquitoes.

d. Vaccination and Immunisation: In this, an antigenic protein of pathogen or inactivated/weakened pathogen (vaccine) is introduced into the body to generate antibodies that neutralise the pathogenic agents during actual infection. A large number of infectious diseases like polio, diphtheria, pneumonia and tetanus have been controlled to a large extent by the use of vaccines.

Benefits of NCERT Solutions

It is highly recommended to refer to NCERT Solutions while preparing for class 12 board exams. Here are some of the benefits of using NCERT Solutions by Extramarks.
1. It is written in such a manner to help the student enjoy the learning journey.

2. It aims to help students grasp the concepts of every chapter.
3. It comes with in-depth explanations to help student boost their confidence.

4. Diagrams are provided, wherever required in the solution.
5. It is free of cost.

Tips & Strategies for Class 12 Exam Preparation

1. Start with making a time table. Prioritize the important topics and study them well.

2. Class 12 is important for your career, therefore follow your time table religiously.

3. Always make brief notes while studying a chapter as they will come in handy for revision before the exam.

4. Understand your concepts, diagrams etc. with NCERT Solutions given on the Extramarks website and the Extramarks – The Learning App.

5. Most importantly, be confident.

Why Opt for Extramarks NCERT Solutions for Class 12 ?

Class 12 board exams are the pillars of a successful career in your life. Thus, with the right study materials, students will be able to achieve their desired marks in exams. NCERT Solutions for class 12 by Extramarks will greatly help students in understanding chapters and will be like a companion in their learning journey.

Frequently Asked Questions

How to study for the Class 12 Maths CBSE exam? 

Math is a subject that analyses the critical and analytical thinking of a student and tests numerical questions. So, the best way to prepare for Math is by studying the NCERT solutions. Make a timetable, jot down the important formulas, and theorems, make time for revision and give as much time as you can give to practicing questions. Solve a maximum number of questions and time your efforts. Extramarks - The Learning App has several sample papers along with NCERT 12 solutions that can be used for practicing for class 12 Math exam.

How to Prepare for Class 12 Board Exams?

Class 12 exams seem like a major feat, but they are actually quite simple and really just a milestone that every student cross in his/her academic life. There is nothing to fear as you can easily prepare for the exams with the help of NCERT solutions for class 12 that are given on the Extramarks website or Extramarks – The Learning App

What part of the CBSE Class 12 exam syllabus is covered in the NCERT books?

The CBSE guide for class 12 study material NCERT contains all syllabus prescribed to students of class 12. Look for NCERT solutions on the Extramarks website in the footer section and you will find all solutions there. 

Do you provide solutions for All subjects for class 12 CBSE? 

Yes, Extramarks provides all NCERT class 12 solutions for all subjects for class 12. Extramarks - The Learning App also has solved and unsolved sample papers that you can use to practice for your exams. You can also find the previous year`s solved board question paper on the app.

What are some expert tips to score good marks in Class 12 CBSE?

To score good marks in class 12 CBSE board exams, you must follow these tips:

1. Make a timetable to study well. Organize and prioritize the topics you want to study and haven`t yet had the time to open. Start studying with the most crucial topics.

2. Follow your timetable religiously. Save time for relaxing activities like meditation, swimming or sleeping.

3. Make brief notes containing important answers, character sketches, theorems, formulae, etc. Make clear notes so you can study them before the exam.

4. Learn from class 12 NCERT solutions given on Extramarks website.

5. Be confident that you can crack these exams and take time off to relax.

6. Revise thoroughly before the exam.

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