NCERT Solutions for class 12th Biology Chapter 8 Human Health and Disease


Question 1. What are the various public health measures, which you would suggest as a safeguard against infectious diseases?

Sol. Public health measures are preventive measures which are taken to check the spread of various infectious diseases. These measures should be taken to reduce the contact with infectious agents. Minimising- the transmission of infectious diseases is a core function of public health law.

The various public health measures against infectious diseases include the following-

  • Education – People should be educated about infectious diseases so that they may protect themselves against the infections.
  • Isolation -A person suffering from any infection should be isolated to avoid its transmission to any other person. Isolation can reduce the chances of spreading airborne diseases such as pneumonia, chicken pox, tuberculosis etc.
  • Sanitation – Sanitary surroundings can prevent the spread of diseases. Maintenance of personal and public hygiene is one of the most important methods of preventing infectious diseases. Public hygiene includes – suitable disposal of waste & human excreta; periodic cleaning and disinfection of water sources; and observing of normal practices of hygiene in public catering. Personal hygiene includes keeping the body clean, intake of clean drinking water, consumption of healthy and nutritious food, vegetables, fruits etc.
  • Vaccination – It is the protection of the body against communicable diseases. It is a process ofinjecting killed microbe in order to stimulate the immune system against the microbe, thereby preventing disease. It helps in providing passive immunization to the body. Several vaccines are available against many diseases like tetanus, polio, measles, mumps, etc.
  • Vector eradication – Vector is an organism that transmits a disease or parasite from one animal to another. It is a carrier of several diseases like malaria, filariasis, dengue, and chikungunya etc. These diseases can be prevented by providing a clean environment and by preventing the breeding of mosquitoes. This can be achieved by not allowing water to stagnate around residential areas.

Also, measures like regular cleaning of coolers, use of mosquito nets and insecticides (such as Malathion) in drains, ponds, etc. can be undertaken to ensure a healthy environment. Introducing laxvivorous fish such as Gambusia in ponds also controls the breeding of mosquito larvae in stagnant water.

Question 2. In which way has the study of biology helped us to control infectious diseases?

Sol. Infectious diseases are disorders caused by pathogenic organisms, such as bacteria, viruses, fungi or parasites. Many organisms which live in and on our bodies are normally harmless or even helpful, but under certain conditions, some organisms may cause disease.

These infectious diseases can be spread through various means, like

  • From person to person.
  • By bites from insects or animals.
  • By ingesting contaminated food or water or being exposed to organisms in the environment.

Each infectious disease has its own specific signs and symptoms. General signs and symptoms common to a number of infectious diseases include : fever, diarrhoea, fatigue, muscle aches, coughing etc.

The study of biology helped us to control these infectious diseases because

a particular branch of biological science (called pathology) helps us to study the causes and effects of disease by examining the samples of body tissue. These examinations can be done for diagnostic or forensic purposes: Advancements achieved in biological science also helped us gain a better understanding of fighting against various infectious diseases and understanding various aspects of life, like the shape, structure and function of living organisms, the evolution of life, etc.

Methods of biological science to control infectious diseases can be done by studying the following:

  1. Life cycle of parasites, etc : With respect to human health, it helped us study the life cycle of various parasites, pathogens, and vectors along with its modes of transmission and the measures for controlling them.
  2. Vaccine: The discovery of vaccines and their timely use in immunization programs have helped in controlling and even eradicating certain infectious diseases. These programs against several infectious diseases such as smallpox, chicken pox, tuberculosis, etc. have helped eradicate these diseases.
  3. Drugs and antibiotics: Biotechnology has also helped in the preparation of newer and safer drugs and vaccines to cure disorders. The discovery of antibiotics and other drugs has also played an important role in treating infectious diseases.

Question 3. How does the transmission of each of the following diseases take place?

(a) Amoebiasis  (b) Malaria  (c) Ascariasis  (d) Pneumonia

Sol. (a) Amoebiasis

  • It is a disease caused by the parasite Entamoeba histolytica.
  • Mode of transmission
    • It is transmitted by the fecal-oral route.
    • It is also transmitted indirectly through contact with dirty hands or objects.
    • Through ingestion of the cyst form (called amoebic cyst) of the parasite, a semi-dormant and hardy structure found in feces.
    • The vector involved in the transmission is the housefly.

(b) Malaria

  • It is a life-threatening mosquito-borne blood protozoan disease, caused by Plasmodium species.
  • Mode of transmission
    • Bite by an infected female mosquito of the genus Anopheles, which previously had stung a person infected with malaria.
    • The mosquito picks up the parasite along with the blood when it bites an infected person. When this mosquito bites another healthy person, the parasites migrate into his blood with the saliva, which the mosquito injects before sucking up blood to prevent its clotting.

Note: Less common ways of transmission of malaria are: blood transfusion, transplanta­tion, infected needles, from mother to fetus during pregnancy.

(c) Ascariasis

  • It is a roundworm infection, caused by Ascaris lumbricoides.
  • Mode of transmission: Through contaminated soil mixed with human feces that contain ascariasis eggs or infected water.

(d) Pneumonia

  • A variety of organisms, including bacteria, viruses, and fungi, can cause pneumonia. The most common are bacteria and viruses in the air we breathe. The most common cause of bacterial pneumonia is Streptococcus pneumoniae. Haemophilus influenza type b (Hib) is the second most common cause of bacterial pneumonia.
  • Mycoplasma pneumonia also can cause pneumonia.
  • Mode of transmission
    • The viruses and bacteria that are commonly found in a child’s nose or throat can infect the lungs if they are inhaled.
    • They may also spread via air-borne droplets from a cough or sneeze.
    • In addition, pneumonia may spread through blood, especially during and shortly after birth.

Question 4. What measures would you take to prevent water-borne diseases?


  • Water-borne diseases are any diseases caused by drinking water contaminated by human or animal feces, which contain pathogenic microorganisms.
  • Examples of waterborne diseases are cholera, typhoid, hepatitis B, etc.
  • Waterborne diseases can be prevented by the following:
  • Oral Rehydration Solution (ORS): It is used to prevent dehydration and electrolyte loss. It is a special combination of dry salts that is mixed with safe water. It can help reduce the fluids lost due to diarrhea.
  • Health education
  • General hygiene, pure water: Water bodies, ponds, and water tank reservoirs in the city or village should be cleaned periodically.
  • Vaccination and immunisation : Vaccination is an effective method to control the outcome of water-borne diseases. For example, two oral cholera vaccines were used in mass vaccination campaigns. Today, they are considered an effective tool against high-risk cholera and are being implemented worldwide.
  • By ensuring proper disposal of sewage excreta, and periodic cleaning.
  • Do not drink untreated water from a spring, river, stream, lake, pond etc.
  • Personal hygiene: A personal level, practice good hygiene such as frequent hand washing and disinfection of cutlery, cutting boards, etc. Wash hands thoroughly and frequently using soap, in particular after contact with pets or farm animals, or after having been to the toilet.

Note: Water-borne disease is easily transmitted when contaminated water is used for any of the following purposes: drinking, making ice cubes, washing uncooked fruits and vegetables, making baby formula, brushing teeth and washing dentures or contact lenses.

Question 5. Discuss with your teacher what does ‘a suitable gene’ means, in the context of DNA vaccines.

Sol. DNA vaccines are third-generation vaccines. They are made up of small, circular pieces of bacterial DNA, plasmids that are genetically engineered to produce specific antigens from a pathogen. After administration of the DNA, antigens are produced and stimulate an immune response. Similarly, a suitable gene in the context of DNA vaccines is the specific segment of DNA which injected into the host in the form of the vaccine. This segment has the ability to produce specific proteins.

This gene which encodes a protein associated with the target infectious organism, is spliced into plasmid, then copied & formulated as a vaccine. This protein acts as an antigen, kills the specific disease-causing organism in the host body, and provides immunity against the disease.

Question 6. Name the primary and secondary lymphoid organs.

Sol. Lymphoid organs are those organs of the immune system, in which lymphocytes can differentiate and proliferate. They are part of the lymphatic system which defends the body against invading pathogens that cause infections or spread of tumors. These organs exist as primary, secondary or tertiary based on the stage of lymphocyte development and maturation the organ is involved in.

  • The primary lymphoid organs consist of specialized tissue, in which generation and maturation of B- and T-lymphocytes takes place. Primary lymphoid organs are bone marrow and thymus. Both bone marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes.
  • Secondary lymphoid organs provide the sites for the interaction of lymphocytes with the antigen, which then proliferate to become effector cells. Secondary lymphoid organs are spleen (white pulp of spleen), lymph nodes and MALT (tonsils, Peyer’s patches of small intestine and appendix).

Note: Tertiary lymphoid organs usually contain less number of lymphocytes. It plays an important role in inflammation. Inflammation protects the body by removing harmful stimuli and initiating a healing process.

Question 7. 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

Question 8. Differentiate the following and give examples of each.

  • Innate and acquired immunity
  • Active and passive immunity

Sol. (a)  Difference between innate and acquired immunity

Question 9. Draw a well-labeled diagram of an antibody molecule.

Sol. Antibodies are globular proteins belonging to the immunoglobulin (lg) family. This structure consists of two identical light (L) chain polypeptides of about 22000 Da and two identical heavy (H) chains of larger polypeptides of about 55000 Da or more. Hence, an antibody is represented as H2L2.

Each light chain is bound to a heavy chain by a disulfide bond and by non­ covalent interactions such as salt bride, hydrogen bonds, and hydrophobic interaction to form a heterodimer (H-L). Similar non-covalent interaction and disulfide linkage link the two identical heterodimers (H-L) to each other from the basic structure of the antibody i.e. dimer of the dimer.

Note: There are five classes of immune globulin-IgA, IgD, Jg£, IgG, and IgM-defined by the amino acid sequence of the heavy chain. They have different roles in immune responses; IgG, IgA, and IgM are commonly produced after viral infection.

Question 10. What are the various routes by which transmission of human immunodeficiency virus takes place?

Sol. HIV (Human immunodeficiency virus) is a virus that targets and alters the immune system by attacking immune cells called CD4 cells, which are a type of T cell, thereby increasing the risk and impact of other infections and diseases. Without treatment, the infection might progress to an advanced disease stage called AIDS (Acquired Immune Deficiency Syndrome). AIDS, the most advanced stage of HIV infection, describes a set of symptoms and illnesses that happen at the final stage of HIV infection if left untreated. HIV is found in semen, blood, vaginal and anal fluids, and breast milk. HIV can’t be transmitted through sweat, saliva or urine.

Various routes by which transmission of human immunodeficiency virus takes place are:

  • Illicit (unprotected or illegal) sexual contact.
  • Sexual contact with multiple partners.
  • Transfusion of the blood of an infected person.
  • Intravenous drug users who share needles are at high risk of contracting AIDS.
  • From an infected mother to a child through the placenta.

Note: Mosquitoes can’t transmit HIV because, unlike mosquito-borne diseases, HIV is unable to replicate within  the mosquito’s gut  and  therefore is broken down. In humans, HIV binds to T cells and begins replicating. No T cells exist inside the mosquito’s gut and so the virus has no way of replicating or migrating to the mosquito’s salivary glands. HIV particles are therefore digested by the mosquito alongside the actual blood meal. During the digestion process, the HIV particles are “completely destroyed.”

Question 11. What is the mechanism by which the AIDS virus causes deficiency of immune system of the infected person?

Sol. AIDS (Acquired Immune Deficiency Syndrome) is caused by the Human immunodeficiency virus (HIV). The HIV virus enters the human body by various methods like sharing infected needles, unprotected sexual contact with an infected person, or infected blood transfusion. HIV critically injures the immune system by infecting and eventually killing T-cells (CD4 T-cells). As the CD4 T-cell counts wane, the body becomes progressively more susceptible to opportunistic infection with intracellular microbes.

Mechanism by which the AIDS virus causes deficiency of immune system

  • After entering the human body, the HIV virus attacks and enters the macrophages. Inside the macrophages, the RNA of the virus replicates with the help of the enzyme reverse transcriptase and gives rise to viral DNA.
  • This process is called reverse transcriptase because it violates the usual way in which genetic information is transcribed. Because reverse transcriptase lacks the proofreading function that most DNA synthesizing enzymes have.
  • Then, this viral DNA is incorporated into the host DNA and directs the synthesis of virus particles. At the same time, HIV enters helper T-lymphocytes where it replicates and produces viral progeny. These newly formed progeny viruses get released into the blood, attacking other healthy helper T-lymphocytes in the body. As a result, the number of T-lymphocytes in the body of an infected person decreases progressively, thereby decreasing the immunity of a person.
  • Death ultimately results from the relentless attack of opportunistic pathogens or from the body’s inability to fight off malignancies.

Note: Infected cell can survive while viruses are being replicated and released.

Question 12. How is a cancerous cell different from a normal cell?

Sol. The main difference between cancer cells and normal cells is that the cancer cells have an uncontrolled growth and cell division whereas the growth and cell division of normal cells is controlled.

Difference between cancerous cells from normal cells is given below:


  • Cancer cells do not have a regular life span.
  • Not all tumours are cancerous.

Question 13. Explain what is meant by metastasis.

Sol. Metastasis is a pathological process of spreading cancerous cells from their primary location (the organ in which the cancer began) to the different parts of the body. The property of metastasis is exhibited by malignant tumours.

These metastatic cells divide uncontrollably, forming a mass of cells called tumour. From the tumour, some cells get sloughed off and enter into the blood stream. From blood stream these cells reach distant parts of the body and therefore, initiate the formation of new tumours by dividing actively.

Note: Cancers can metastasise to nearly any regions of the body, but some of the common sites are the bones, lungs, liver, and brain.

Question 14. List the harmful effects caused by alcohol/drug abuse.

Sol. Alcohol and drugs have several adverse effects on the individual, his family and the society.

  • Harmful effects caused by alcohol abuse
    • Alcohol generates more energy mostly in the form of heat, but at the same time, it dilates the blood vessels. Consequently the heat generated is rapidly lost. Due to constant dilation, the arterial walls soon become brittle & rigid. Such a change in the property of blood vessels & deposition of alcoholic fat affect the working of heart.
    • Alcoholism leads to gastric ulcers & gastritis.
    • In chronic alcoholism, the axon of the nerve inflame thus causing neuritis.
    • Permanent damage to liver cells occur due to deposition of fats. The liver dries up & harden (cirrhosis).
    • Consumption of excess alcohol by any family member can have devastating effects on the family. It leads to several domestic prob­ lems such as quarrels, frustrations, insecurity, etc.
  • Harmful effects caused by drug abuse
    • Excessive doses of drugs may lead to coma & death due to respiratory failure, heart failure & cerebral haemorrhage.
    • It will lead to lack of interest in personal hygiene, withdrawal, isolation, depression, fatigue aggressive & rebellious behaviour etc. Acquire serious infections like AIDS & hepatitis B, who take drugs intravenously.
    • The adverse effects of drugs are manifested in the form of reckless behaviour, vandalism & violence.

Question 15. 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 alcohols.
  • A person can take the following steps for protecting himself against drug abuse:
    • Increase your will power to stay away from alcohol and drugs. One should not experiment with alcohol for curiosity and fun.
    • Avoid the company of friends who are addicted to drugs and alcohol.
    • Seek help from parents and peers.
    • Take proper knowledge and counseling about drug abuse.
    • Devote your energy in other extra-curricular activities.
    • Not take undue pressure of failures beyond its threshold.
    • Seek immediate professional and medical help from psychologists and psychiatrists if symptoms of depression and frustration become apparent.

Question 16. 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.

Sol. Drug and alcohol consumption has an inherent addictive nature associated with euphoria and a temporary feeling of well-being. Repeated intake of drugs increases the tolerance level of the body’s receptors, leading to more consumption of drugs. Psychological & physiological dependence of an individual to the intake of certain kinds of drugs and alcohol is called addiction.

Once a person start taking alcohol & drugs, it is very difficult to get rid this habit because of the following reasons:

  • People with addiction lose control over their actions. They crave and seek out drugs, alcohol, or other substances no matter what the cost­ even at the risk of damaging friendships, hurting family, or losing jobs.
  • Addiction drive people to take them even when these are not needed or even when their use becomes self destructive.
  • With repeated use of drugs, the tolerance level of the receptors present in the body increases, consequently the receptors responds only to higher doses of drugs or alcohol leading to greater intake & addiction. Thus, the addiction potential of drugs & alcohol, pull the user into a vicious circle leading to their regular use (abuse) from which he/she may not able to get out.

Note: Addiction is a disease that affects the brain, but that doesn’t mean anyone is helpless. The brain changes associated with addiction can be treated and reversed through therapy, medication, exercise, and other treatments.

Question 17. In your view what motivates youngsters to take to alcohol or drugs and how can this be avoided?

Sol. Factors which motivates youngsters to take alcohol or drugs : Many factors are responsible for motivating youngsters towards alcohol or drugs, like modern society, curiosity, need for adventure and excitement, experimentation.

  • Several mediums like television, internet, newspaper, movies etc. are also responsible for promoting the idea of alcohol to the younger generation. Amongst these factors, reasons such as unstable and unsupportive family structures and peer pressure can also lead an individual to be dependent on drugs and alcohol.
  • Human have probably been using mind – affecting drugs since time immemorial. The root cause of addiction of man to drugs, smoking and drinking has been due to his inability to make mental adjustments with stresses and strains, drudgery and extreme misery in daily life. As a temporary measure, to combat these adverse situations and to have a certain degree of mental relaxation, humans have been making an extensive use of stimulants, depressants and hallucinogens. Stimulants generally speed up body process, and depressants slow them. Hallucinogens can alter a person’s thoughts, feelings, and perception.

Preventive measures against addiction of alcohol and drugs

  • Parents should motivate and try to increase the will power of their child.
  • Parents should educate their children regarding the harmful effects of alcohol and drug abuse and provide proper knowledge and counseling regarding the consequences of addiction to drug and alcohol.
  • Youngsters should be kept away from the company of friends who consume drugs.
  • Children should be encouraged to devote their energy in other extra- curricular and recreational activities.
  • Regular and proper professional and medical consultation should be provided to a child if sudden symptoms of depression and frustration are observed.

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