Class 9 Science Chapter 13 Important Questions of Why do we fall ill. All the questions are helpful in the preparation of school exams as well as revising the concepts of the chapter. Class 9 Science Chapter 13 Extra Questions Answers contains all the intext questions as well as exercises questions with suitable answers and explanation.

Class 9 Science Chapter 13 Important Questions for 2022-2023

Class 9 Science Chapter 13 Important Extra Questions Set – 1

What is meant by the symptoms of a disease?

The conditions in which a stick person experience headache, watering of eyes, loose motions are collectively called the symptoms of a disease.

Define reservoir.

A reservoir is defined as any person, animal, arthropod, plant, soil or substance in which an infectious agent lives and multiplies for its primary survival.

What is a source of infection?

Person, animal, object or substance from which an infectious agent passes or is disseminated to the host is known as the source of infection.

How can we diagnose a disease?

By laboratory tests.

What is a community health?

It is the personal health along with the environmental services for the importance of health of the community.

What does WHO stand for?

World health Organisation.

How can dehydration of the body be prevented?

Dehydration can be prevented by intake or ORS.

What are the constituents of a balanced diet?

Constituents of a balanced diet are as follow:

    • Carbohydrate: It provides 50-70% of total energy intake.
    • Fat: For an adult fat should provide 20% of total energy intake. Children require more fat so as to suffice 50% of the total energy intake.
    • Proteins: Protein intake should be about 15-20%of the total daily energy intake.
    • Vitamins and minerals
    • Water
    • Roughage

Class 9 Science Chapter 13 Important Extra Questions Set – 2

How does WHO define health?

WHO defines health as a state of complete physical, mental and social well-being and not merely an absence of disease or infirmity.

Classify diseases on the basis of their time of occurrence.

Congenital diseases and acquired diseases.

What are congenital diseases?

The diseases which are present from the time of birth and are hereditary are called congenital diseases.

Write the names of a diseases that spreads through direct contact.


Why is rabies also called as hydrophobia?

Rabies is also called as hydrophobia because its main symptom is fear of water.

Name the disease-causing microbe that lives and remains active inside the host cell.


Against what disease BCG vaccine is given?


What is the difference between bring healthy and diseases free?

A person is said to be healthy when:

    1. All the organs and systems of the body are intact and working well.
    2. One is mentally balanced, free from anxieties and tensions.
    3. One is socially well- adjusted in the family, friend and society.

    Whereas being diseases free mean absence of any body discomfort. Thus being healthy is not just freedom from diseases.

Class 9 Science Chapter 13 Important Extra Questions Set – 3

What are vector.

The organisms that act as intermediaries and carry the infectious agents from a sick person to a potential host are called vectors.

Name the vector of malaria.

Female Anopheles mosquito.

What do you mean by immune system?

The system in our body which protects us from the various disease-causing agents is called immune system.

Expand AIDS?

Acquired Immuno Deficiency Syndrome.

Expand HIV?

Human Immunodeficiency Virus.

What is the incubation period of hepatitis B?

45 to 185 days.

How can aids be transmitted?

AIDS can be transmitted through sexual intercourse, use of contaminated syringes as well as by transfusion of contaminated blood.

Symptoms of Malaria

    1. Sudden Appearance of fever with pain and sensation of cold shivering.
    2. Body temperature rises up to 106% F and patient becomes burning hot. He or she experiences intense headache, faster breathing rate and heartbeat.
    3. Fever later comes down with profuse sweating. This occurs either daily at a particular time or is repeated every third or fourth day depending upon the species of the parasite.
    4. Enlargement of spleen and anaemia occurs.

Class 9 Science Chapter 13 Important Extra Questions Set – 4

When is World AIDS Day observed?

On 1st December.

What is immunity?

Immunity is the ability of the body to defend against a disease.

Deficiency of which vitamin causes xerophthalmia.

Vitamin A.

Expand OPV.

Oral Polio Virus vaccine.

Give the name of two diseases which results from protein malnutrition.

Kwashiorkor and marasmus.

What is the cause of disease beri-beri?

Deficiency of water-soluble vitamin B1 cause beri-beri.

Give the full form of PEM?

Protein Energy Malnutrition.

Deficiency Diseases

Deficiency diseases are caused due to deficiency of certain nutrients in our diet like proteins, minerals and vitamins. It is a type of non-communication diseases. Examples: Kwashiorkor, marasmus, anaemia, etc.

Class 9 Science Chapter 13 Important Extra Questions Set – 5

What is the functions of haemoglobin?

Transport of oxygen from lungs to all cells of the body through blood.

Name the mineral present in haemoglobin.


Name the fat-soluble vitamins.

Vitamin A, D, E and K are fat soluble.

Name the disease caused by the deficiency of iodine.


While going abroad why is it essential to get vaccinated against certain diseases?

A person may be a carrier of a diseases, such a person can carry the diseases to a foreign country. To avoid the person is vaccinated.

What is the method of transmission of the disease cholera?

Contaminated water

Which vitamin is formed by the body with the help of sunlight?

Vitamin D.

Degenerative Diseases

Degenerative diseases are caused due to malfunctioning of body organs or degeneration of tissues in old age. It is a type of non-communicable diseases. Example: Kidney failure is due to improper functioning of kidneys; cancer is due to uncontrolled growth of tissue in any part of the body etc.

Class 9 Science Chapter 13 Important Extra Questions Set – 6

Name the disease caused by the deficiency of vitamin D?


Name the diseases which causes swollen and bleeding gums.


What is the name of the disease that occurs in people who eat polished rich every day?


Why should not vegetables and pulses be washed repeatedly for a long time?

Repeated washing of vegetables and pulses results in depletion of vitamin C which is a water-soluble vitamin.

What is an antibiotic? Give two examples.

Antibiotic is a chemical substance secreted by microorganisms which can kill bacteria. For example, penicillin and streptomycin.

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

Bacteria and fungi

Who discovered vaccine for the first time? Name the diseases which can be preventing by using vaccines.

Edward Jenner discovered vaccine for the first time. Small pox and polio can be prevented by using vaccines.

What is a balance diet?

A balanced diet is the one which contains a variety of foods in such quantities and proportions that the need for energy, amino acids, vitamins, minerals, water and roughage is adequately met for maintaining health, vitality and general well-being.

Class 9 Science Chapter 13 Important Extra Questions Set – 7

Why is immune system essential for our health?

The immune system of our body is a defence mechanism to fight against pathogenic microbes. It has cells that are specialised to kill infecting microbes and keep our body healthy. If the body’s immune system is strong, it can easily fight pathogens, keeping us healthy.

Why is mother’s milk best for babies?

Mother’s milk is rich in proteins and other nutrients. It provides a complete diet to the baby. Intake of mother’s milk increase body weight, body muscles and subcutaneous fat.

What are the two basic principles of preventions of infectious diseases?

The two basic principles of prevention of infectious diseases are:
(i) Prevention from exposure to infectious microbes.
(ii) Provision of proper nutrition to keep the immune system in a healthy state.

What is a mechanism of action of antibiotics?

Antibiotics are chemical substances obtained from some microbes, which stop the growth of specific kind of pathogens. They block certain biochemical pathways important for the lifecycle of pathogen. For example, penicillin does not allow cell wall formation in some bacteria. It blocks chemical reaction required for cell wall formation.

Public cleanliness is important for individual health; Comment.

The garbage thrown in open places, overflowing drains or sewer water, stagnant water, etc. are the places where disease-causing microbes multiply and mosquitoes and flies breed. These mosquitoes and flies act as carriers of disease-causing microbes. As a result, diseases may spread in the community and affect individual health. Thus public cleanliness is important for individual health.

Why is social equality necessary for individual health?

If the mind is cheerful and happy, people are not prone to tensions. Moreover, as they are not disturbed, they will take care about doing anything which affects their health. On the other hand, if people are socially disturbed and unsafe, they cannot be happy and healthy. So, social equality is necessary for individual health.

Why are good economic conditions needed for individual health?

First of all, for good health, proper and sufficient food is necessary. This food can be obtained only by spending money, for which the individual has to earned. So, opportunity to work and earn have to be made available for which there must be good economic conditions in the society.

The Cause of Diseases

Disease may be caused due to any of the following reasons:

    • (i) Infection
    • (ii) Lack of nutritive and sufficient food.
    • (iii) Poor health
    • (iv) lack of public services
    • (v) Hereditary reasons

Class 9 Science Chapter 13 Important Extra Questions Set – 8

Why is it considered important to study the different categories of infectious agents?

The infectious agents have been categorised as these categories are important factors in deciding the kind of treatment to be used to treat the diseases caused by them.

Why there is no use of giving vaccine of hepatitis A virus?

As the majority of children in many parts of Indian are exposed to the virus causing hepatitis A, they are already immune to hepatitis A by the time they are five years old. So there is no use of giving vaccines.

Why are antibiotics not effective for viral disease?

Antibiotics act by inhibiting the biosynthesis pathways. This way they eventually die. However, viruses do not have components for the biosynthetic pathways. Instead they utilise the components of the host body to complete their life cycles. Therefore, antibiotics are not affective against viruses.

(i) Which bacterium causes peptic ulcers? (ii) Who discovered the above pathogens for the time?

(i) Helicobacter pylori.
(ii) Marshall and Warren.

What do you mean by disease symptoms? Explain giving two examples.

When the functioning or the appearance of one or more systems of the body will change for the worse, it gives certain abnormal signs of the disease. These visual changes in human being are called symptoms. Symptoms give indication of the presence of a particular disease.
Examples: (i) Cough is the symptoms of lung infections.
(ii) Lesions on the skin are the symptoms of chicken pox.

Becoming exposed to or infectious microbe does not necessarily mean developing noticeable disease. Explain.

Because of strong immune system, our body is normally fighting off microbes. We have cells which are specialised to kill the pathogenic microbes. These cells are active when infecting microbes enter the body and if they are successful in removn the pathogen, we remain diseases free. So even if we are exposed to infectious microbes, it is not necessary that we suffer from diseases.

(i) Name two diseases caused by Protozoa. (ii) What are their causal organism?

(i) Sleeping sickness caused by Trypanosoma and malaria by Plasmodium.
(ii) kala-azar caused by Leishmania.

Why is vaccination considered a prevention of diseases?

Vaccines induce a specific immune response in the body. This response also produces memory cells which persist in the body even in the absence of pathogen. If the pathogen attacks the body again, the immune system with the help of memory cells recognise it and destroy it before it causes the disease.

Class 9 Science Chapter 13 Important Extra Questions Set – 9

Write the distinct species of malarial parasite in man.

Malaria in man is caused by four distinct species of malarial parasites:
(a) Plasmodium vivax: incubation periods in human is 8 – 17days.
(b) P.falciparum: incubation periods in human is 9 – 14 days.
(c) P.malariae: incubation periods in human is 18 – 40 days.
(d) P.ovale: incubation period in human is 16 – 18 days.

What are the immediate and contributory causes of diseases? Explain it with the example of a child suffering from diarrhoea.

Immediate cause of a diseases is the primary factors causing a disease. Contributory causes are factors, which do not cause the diseases themselves but provides conditions for the diseases to occur. Virus causing diarrhoea is the maintain cause. Contaminated drinking water and lack of resistance due to under-nourishment are the contributory causes.

Write the symptoms when following organs are targeted by microbes. (a) Lungs (b) Liver (c) Brain

(a) Lungs – cough, breathless.
(b) Liver – Jaundice
(c) Brain – headache, vomiting, fits.

Describe health care.

Health care is provided to vast majority of poor, rural and urban people through effective health care centers. Health care services provide different types of care at primary health care centre and secondary health care centers.
Primary health care is provided by the primary health centres establishment in small towns and villages, through the agency of health workers, village health guide and trained dhayas.
Secondary health care deals will more complex problems. It is generally provided in district hospitals and community health centers.

What determines the severity of diseases manifestation?

The number of disease-causing microbes in the body decide the severity of disease manifestation. If the number of microbes is very small, the disease manifestation may be minor and even go unnoticed. But if the number of microbes is large, the disease can be severe. In fact in fact if the number of microbes is very large, the disease can even be fatal. Our immune system is a major factor that determines the number of microbes surviving in the body.

Name the infectious diseases that leads to immunodeficiency. Give the scientific name of the pathogens causing the diseases and mention the body organs it primarily affects.

AIDS is an infectious diseases that leads to immune deficiency and wasting of body parts. It is caused by Human Immunodeficiency Virus (HIV). HIV attacks helper T- lymphocytes, thus causing cell mediated immunodeficiency, which makes the body more prone to various infections.

What are the causes and symptoms of goitre?

Goitre is caused due to deficiency of iodine in the diet.
Symptoms of goitre are as follows:
(i) Abnormal growth of thyroid gland situated in the front part of the neck.
(ii) Increase in body weight due to accumulation of fat and retention of water in the body.
(iii) Increased rate of spontaneous absorption and still birth.
(iv) Disorder in nervous system. Iodine deficiency in childhood causes functioning of the thyroid gland resulting in retarded growth.

Congenital Diseases

Congenital disease is the one that is present in an individual from birth. This may be due to genetic abnormality, metabolic disorders or malfunctioning of any body organ. These are permanent and are generally not easily curable. These are passed to the children from parents.

Class 9 Science Chapter 13 Important Extra Questions Set – 10

What are the source of iodine? What are the prevention and control method of goitre?

Source of iodine: The best source of iodine are sea foods and cod liver oil. A smaller amount of the iodine occurs in milk, leafy vegetables, cereals and meat, etc. Iodised salt contain contains sufficient amount of iodine.
Prevention and control goitre: It can be prevented by providing iodine in the diet in the form of iodised salts, such as potassium iodate and potassium iodide. These can be added in drinking water or in common salt used daily. Intra-muscular ingestion of iodised oil or sodium iodide tablets developed by Indian Council of Medical Research is quite effective in curing goitre.

What are the common preventive measures against communicable diseases?

The common preventive measures against communicable diseases include.
(i) Eradication of vectors and carries.
(ii) immunization (vaccination)
(iii) Proper and safe water supply.
(iv) Personal and community hygiene.
(v) Sterilisation of articles used by the patients.
(vi) Isolation of patients from the healthy person.
(vii) Health education.

Name the diseases caused by the following – (i) Protozoa, (ii) Virus, (iii) Bacteria (iv) Fungi (v) How is malaria transmitted?

The diseases caused by various microorganism are follows:
(i) Protozoa: Malaria, amoebiosis, dysentery, giardiasis, kala-azar, etc.
(ii) Virus: AIDS, polio, dengue, rabies, chicken pox, influenza etc.
(iii) Bacteria: Pneumonia, diphtheria, tuberculosis, meningitis, leprosy, typhoid, tetanus, syphilis, etc.
(iv) Fungi: Fungi mainly causes skin diseases and food poisoning.
Malaria is caused by a parasite found in female Anopheles mosquitoes. When the mosquitoes carrying the malaria parasite bite a person, the parasite enters the blood stream and the person suffers from malaria.

What are the three limitation which one has to face while dealing with an infectious disease?

The three limitations which one has to face while dealing with an infectious disease are:
(i) The body functions are damaged drastically and may never recover completely if not cared.
(ii) The treatment will take time which means that someone suffering from a disease is likely to be bed ridden for some time.
(iii) The person suffering from an infectious disease can serve as a sources from where the infection may further spread to other people.

What is immunity? Explain natural and acquire immunity.

Immunity means the resistance of the body to a disease. It is due to the presence of antibodies in our body against the disease-causing microorganism known as antigens. When these antigens enter our body, antibodies are formed which prevent the disease.
Natural immunity means that a person has these antibodies since birth e.g. whenever antigens, say of cholera enter the body, the person will not suffer from the disease.
Acquired immunity means when a person suffers from a disease once antibodies for these particular disease-causing antigens will be formed in the body and he will not get the same disease again.

Give an example where tissue specificity of the infection leads to a very general seeming effects.

We can see the tissue specificity of the infection leading to very general seeming effects in case of HIV infections. The HIV attacks the immune system via the lymph nodes. From here it spreads all over the body and damages its functions. Because of this, the body becomes prone to various diseases as it cannot fight off even the mirror infectious which otherwise would not have lasted longer.
For example, even a small cold can become pneumonia and a major gut infection may lead to a severe case of diarrhoea with blood loss.
In the same way, other infections kill people that are suffering from e.g., HIV-AIDS.
The tissue specificity of the infection (HIV-AIDS) is lymph nodes. General seeming effects are loss of immunity even to minor diseases or infection that ultimately lead to the death of the patient.

What precautions will you take to justify “prevention is better than cure”?

Following precautions should be taken for prevention of diseases:
(i) Maintaining hygienic conditions.
(ii) Awareness about the diseases and its causal organism.
(iii) Intake of a balanced diet.
(iv) Regular medical check-up.

Class 9 Science Chapter 13 Important Extra Questions Set – 11

Give any four factors necessary for a healthy person.

For a healthy person it is necessary that (i) the surrounding environment should be clean. Air and water-borne diseases should not spread.
(ii) personal hygiene is maintained to prevent infectious diseases.
(iii) proper, sufficient nourishment and food is available for good immune system of our body. (iv) body is immunized against severe diseases.

Diseases the causes, symptoms, prevention measures and treatment of AIDS.

AIDS is caused by a retrovirus, HIV (Human Immunodeficiency Virus). It is transmitted from an infected to a healthy person through sexual contact, blood transfusion, use of infected needle or blade. Also, it may get transmitted from infected mother to her foetus.
(i) Inflammation in lymph glands.
(ii) Loss of weight and sweating during night.
(iii) Bleeding and fever.
(iv) Severe damage to the brain which may even lead to loss of memory. In some cases, the person may speak cease to speak and even think.
(v) Above all, the possibility of other diseases also increases because of the damage caused to the immune system.
(i) By avoiding sexual contact with unknown persons.
(ii) By using sterilized needles, blades, etc.
(iii) By ensuring that the blood to be transfused is free from HIV.

What do you mean by diseases? Describe the various causes of diseases.

Diseases is defined as a condition of the body or a part of it in which its normal functioning gets disturbed. The main causes of diseases are:
(i) Biological agents which are disease-causing microorganism or pathogens like viruses, bacteria, fungi, protozoa, etc.
(ii) Physical agents like heat, cold, radiation, humidity, pressure, electricity, sound etc.
(iii) Mechanical agents like such as chronic friction as well as other mechanical forces that cause injuries, trauma and fracture etc.
(iv) Chemical agents which are of two types;
(a) Endogenous chemical agents which are formed in the body, like urea, uric acid, etc.
(b) Exogenous chemical agents which enter the body of the individual from outside, like metals, spores and pollens etc.
(v) Inherited diseases: Some diseases are because of the hereditary reasons, disturbance in chromosomes or genes.
(vi) Social agents which may cause mental disorders.
(vii) Deficiency diseases are caused by insufficiency, absence or excess of a nutrient necessary for health.

Discuss types of anaemia with their symptoms.

Iron deficiency in the body results in anaemia. Nutritional anaemia is a diseases syndrome caused by malnutrition. It is of two types:
(i) Microcytic anaemia
(ii) Pernicious anaemia
Microcytic anaemia: It occurs due to deficiency of iron in human diet.
An anaemic person:
(i) becomes pale, weak and tired
(ii) loses appetite.
(iii) loses body weight.
Pernicious anaemia: It is caused due to deficiency of vitamin B12.
Symptoms: Patient becomes paler, shortness of breath after slight exertion, loss of weight, weakness, etc. It may be fatal.

Why is AIDS considered to be a ‘syndrome’ and not a disease?

AIDS causing virus-HIV that comes into the body via the sexual organs or blood transfusion will spread all over the body through lymph nodes. The virus damages the immune system of the body and due to this body can no longer fight off many minor infections. Instead, every small disease like cold can become severe pneumonia or minor gut infection can become severe diarrhoe with blood loss. The effect of diseases becomes very severe and complex, at times killing the person suffering from AIDS. Hence, there is no specific disease symptom for AIDS but it results in a complex disease. Therefore, it is known as a syndrome.

What are the essential components of primary health care?

Primary care includes the following essential components:
(i) Providing education concerning prevailing health problems and methods of preventing and controlling them.
(ii) Provisions of food supply and proper nutrition.
(iii) Adequate supply of safe and clean drinking water and basic sanitation.
(iv) Provisions of maternal and child health care.
(v) Immunisation against major infectious diseases.
(vi) Prevention and control of local epidemic diseases.
(vii) Appropriate treatment of common diseases and injuries.
(viii) Provisions of essential drugs.
(ix) Promoting health education in schools and colleges.

Describe influenza with its symptoms and prevention.

Influenza or flu is an acute respiratory tract infection caused by influenza virus. It is of three types-A, B and C. Influenza virus A and B have caused epidemics throughout the world. Influenza virus C occurs sporadically in the form of small outbreak. Major reservoirs of influenza virus are animals and wild birds. Influenza is spread mainly from person to person by droplet infection created by sneezing, coughing or talking and enters the respiratory tract. Its incubation period is 18 to 72 hours.
Symptoms: Fever, chills, aches and pain, coughing and weakness.
Prevention: Sufferers should cover their faces with handkerchief while coughing and sneezing. Healthy persons should remain away from such patients

Explain giving reasons: (a) Balanced diet is necessary for maintaining healthy body. (b) Health of an organisms depends upon the surrounding environmental conditions. (c) Our surrounding area should be free of stagnant water. (d) Social harmony and good economic conditions are necessary for good health.

(a) Food is necessary for the growth and development of the body. Balanced diet provides raw materials and required energy in appropriate amount through nutrients like proteins, carbohydrates, fats, minerals, etc. which in turn are essential for the proper growth and functioning of a healthy body.
(b) Health is a state of being well enough to function physically, mentally and socially and these conditions in turn depend upon the surrounding environmental conditions e.g., if there are unhygienic conditions in surrounding area, it is likely that we might get infected or diseased.
(c) This is so because many water-borne diseases and insect vectors flourish in stagnant water which cause disease in human beings.
(d) Human beings live in societies and different localities like village or cities, which determine the social and physical environment and hence both are to be kept in harmony. Public cleanliness is important for individuals health. For better living conditions money is required. We need good food for healthy body and for this we have to earn. For the treatment of diseases of diseases also, one has to be in good economic condition.

Difference between Communicable and Non-communicable Diseases.

Communicable DiseasesNon-communicable Diseases
1. These diseases can be transmitted from an infect person to a healthy person.1. These diseases cannot be transmitted.
2. These are spread by microorganism called pathogens.2. These are caused by deficiency of nutrients or hormone, tumour formation, etc.
3. Example: Cholera, influenza, AIDS, malaria, etc. 3. Example: Diabetes, marasmus, goitre, cancer, etc.