Chapter 13 — Why Do We Fall Ill?
Welcome to HSLC Guru! This page provides complete English-medium notes and question answers for ASSEB Class 9 Science Chapter 13 — Why Do We Fall Ill? Here you will find a clear summary, all textbook questions with detailed answers, additional MCQs, fill in the blanks, true or false, and a glossary table — everything you need to score full marks in your ASSEB Class 9 Science examination.
Summary
Health and Disease: Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity. Disease (literally “dis-ease”) means our body is not at ease — its functioning or appearance has changed in an unpleasant way. The symptoms of a disease are signs like headache, cough, loose motions or wound with pus, which point that there may be a disease, but they do not tell us what the disease is. Personal health depends on physical surroundings, economic conditions, social equality and harmony, public services like clean water, garbage disposal, and a healthy community environment. This is why personal health and community health are deeply linked — an individual cannot remain healthy if the community is unhealthy.
Acute and Chronic Diseases: Diseases that last for only a short period of time are called acute diseases (for example, common cold). Diseases that last for a long time, even as much as a lifetime, are called chronic diseases (for example, elephantiasis, tuberculosis). Chronic diseases have very drastic long-term effects on people’s general health compared to acute diseases. Diseases can also be classified as infectious (caused by infectious agents — microbes such as bacteria, viruses, fungi, protozoans and worms) and non-infectious (caused by genetic factors, deficiency of nutrients, lifestyle, etc.). Examples of infectious diseases include tuberculosis (bacteria), common cold and AIDS (virus), skin diseases (fungi), malaria and kala-azar (protozoans), and elephantiasis and ascariasis (worms). Examples of non-infectious diseases include cancer, high blood pressure, diabetes and genetic disorders.
Spread of Diseases — Means of Transmission: Infectious diseases (also called communicable diseases) spread from an infected person to a healthy person through several routes. They spread through air (droplets while sneezing or coughing — common cold, pneumonia, tuberculosis), through water (contaminated drinking water — cholera, typhoid, hepatitis A), through food (food contaminated with microbes), through vectors like mosquitoes (malaria by Anopheles, dengue by Aedes), through sexual contact and blood (AIDS, syphilis, hepatitis B), and from mother to child during pregnancy or breastfeeding (AIDS). Common diseases that students must know are tuberculosis (TB), cholera, typhoid, malaria, AIDS, hepatitis and dengue. Each microbe enters the body through a specific route, multiplies in a specific tissue, and produces specific signs and symptoms.
Principles of Treatment and Prevention: Treatment of an infectious disease has two main aspects — (i) reducing the effects of disease by giving medicines to bring down symptoms such as fever, pain or loose motions, and taking complete rest, and (ii) killing the cause of the disease by using drugs (antibiotics for bacteria, antiviral drugs for viruses) that block important biochemical pathways of the microbe without harming our own cells. Antibiotics like penicillin work because they block the bacterial process of making cell walls; they do not work against viruses because viruses do not have their own biochemical machinery. Prevention is always better than cure. There are two ways of prevention — general (clean drinking water, proper sanitation, good nutrition, clean surroundings, vector control) and specific (vaccination/immunisation). Vaccines work on the principle of immune memory — our immune system remembers a previous infection and produces a strong response when the same microbe attacks again. Vaccines are available against tetanus, diphtheria, whooping cough, measles, polio, hepatitis B, etc. A healthy immune system, supported by good nutrition, is the body’s best defence against most infections.
Textbook Question Answers
1 Mark Questions
Q1. Define health.
Answer: Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.
Q2. What is a disease?
Answer: Disease means being uncomfortable — a condition in which the normal functioning or appearance of the body is disturbed due to some cause, producing signs and symptoms.
Q3. Name two acute diseases.
Answer: Common cold and influenza (flu) are acute diseases.
Q4. Name two chronic diseases.
Answer: Tuberculosis and elephantiasis are chronic diseases.
Q5. Name the causative organism of tuberculosis.
Answer: Tuberculosis is caused by the bacterium Mycobacterium tuberculosis.
Q6. Name the vector that spreads malaria.
Answer: The female Anopheles mosquito is the vector that spreads malaria.
Q7. Which mosquito spreads dengue?
Answer: Dengue is spread by the female Aedes aegypti mosquito.
Q8. What is the full form of AIDS?
Answer: AIDS stands for Acquired Immuno Deficiency Syndrome. It is caused by the HIV virus.
Q9. Name one disease that spreads through contaminated water.
Answer: Cholera is a disease that spreads through contaminated drinking water.
Q10. What is a vaccine?
Answer: A vaccine is a biological preparation made of weakened or killed microbes that, when given to a healthy person, makes the immune system produce protective memory against that microbe.
2-3 Marks Questions
Q1. Differentiate between acute and chronic diseases with examples.
Answer: Acute diseases last for only a short period of time. They do not have very long-lasting effects on the health of the patient. Example — common cold, which lasts for a week. Chronic diseases last for a long time, sometimes for a lifetime. They have drastic long-term effects on the patient’s general health and ability to work. Examples — tuberculosis and elephantiasis. A person with a chronic disease loses weight and becomes weak, while an acute disease usually has no such effect after recovery.
Q2. Differentiate between infectious and non-infectious diseases.
Answer: Infectious diseases are caused by microbes (infectious agents) such as bacteria, viruses, fungi, protozoans or worms. They can spread from one person to another through air, water, food, vectors or contact. Examples — tuberculosis, cholera, malaria, AIDS. Non-infectious diseases are caused by internal factors such as genetic abnormalities, nutritional deficiency, hormonal imbalance or lifestyle. They do not spread from person to person. Examples — cancer, diabetes, high blood pressure, goitre.
Q3. Why is personal health linked to community health?
Answer: Personal health depends on the physical and social environment in which a person lives. If the surroundings are dirty, drinking water is contaminated, garbage is not collected, and the community has no proper sanitation, even a careful individual will fall ill. Mosquitoes breeding in the neighbourhood will bite everyone; an infected person can spread disease through air or water to all. Therefore, no person can remain healthy unless the whole community has good public hygiene, clean water, proper food, social harmony and economic well-being.
Q4. Mention three ways in which infectious diseases spread.
Answer: (i) Through air — when an infected person sneezes or coughs, tiny droplets containing microbes are released which a healthy person inhales (common cold, tuberculosis, pneumonia). (ii) Through water — drinking water contaminated with the excreta of an infected person spreads diseases like cholera, typhoid and hepatitis A. (iii) Through vectors — animals such as mosquitoes carry microbes from a sick person to a healthy person, spreading malaria, dengue and kala-azar.
Q5. Why are antibiotics not effective against viral diseases?
Answer: Antibiotics work by blocking biochemical pathways important for bacteria, such as the synthesis of bacterial cell walls. Viruses do not have cell walls and do not have their own biochemical machinery — they live inside human cells and use the host cell’s machinery for their life processes. Since antibiotics cannot find any bacterial pathway in viruses to block, they are ineffective against viral diseases. Antiviral drugs are needed to treat viral infections, but they are difficult to make.
Q6. What are the principles of prevention of infectious diseases?
Answer: There are two principles of prevention. (i) General methods — these aim at preventing exposure to infectious microbes by providing safe drinking water, proper sanitation, clean surroundings, hygienic food, control of vectors like mosquitoes, and good nutrition that keeps the immune system strong. (ii) Specific methods — these aim at boosting the immune system through vaccination/immunisation, where vaccines made of weakened or killed microbes prepare the body to fight specific diseases like tetanus, polio, measles, hepatitis B, tuberculosis (BCG), etc.
5-6 Marks Questions
Q1. Explain the different types of infectious agents that cause diseases in humans, with one example of a disease caused by each.
Answer: Five major types of infectious agents (microbes) cause diseases in humans:
- Bacteria: Single-celled prokaryotic organisms. Cause diseases such as tuberculosis (Mycobacterium tuberculosis), cholera (Vibrio cholerae), typhoid (Salmonella typhi) and anthrax. Treated with antibiotics.
- Viruses: Acellular agents that multiply only inside living host cells. Cause common cold, influenza, AIDS (HIV), dengue, hepatitis, polio and measles. Antibiotics do not work; antiviral drugs and vaccines are used.
- Fungi: Eukaryotic organisms that cause diseases such as ringworm, athlete’s foot and many skin infections, especially in moist conditions.
- Protozoans: Single-celled eukaryotes. Cause malaria (Plasmodium), kala-azar (Leishmania), amoebic dysentery (Entamoeba histolytica) and sleeping sickness (Trypanosoma).
- Worms (Helminths): Multicellular parasites such as Ascaris (causing ascariasis), tapeworms and Wuchereria (causing elephantiasis or filariasis).
Drugs against each group target a different biochemical step, because organisms in different groups differ greatly in their structure and biochemistry.
Q2. Describe the different means by which infectious diseases are transmitted, giving examples for each.
Answer: Infectious diseases (communicable diseases) spread from sick persons to healthy persons by the following means:
- Through air: When a sick person sneezes or coughs, droplets carrying microbes are thrown into the air. A healthy person inhales them and gets infected. Examples — common cold, pneumonia, tuberculosis, COVID-19.
- Through water: When excreta of a sick person mix with drinking water sources, the microbes enter healthy persons through drinking. Examples — cholera, typhoid, hepatitis A, amoebic dysentery.
- Through food: Eating food contaminated by flies or unhygienic handling causes diseases such as gastroenteritis and food poisoning.
- Through vectors: Animals (mostly insects) act as carriers. Female Anopheles mosquito carries malaria parasite; female Aedes carries dengue and chikungunya; sandflies carry kala-azar.
- Through sexual contact and blood: Diseases like AIDS, syphilis and hepatitis B spread through unprotected sexual contact and through transfusion of infected blood or use of unsterile needles.
- From mother to child: AIDS and some other diseases pass from an infected mother to her baby during pregnancy, childbirth or breastfeeding.
Knowledge of these routes helps us prevent diseases by breaking the chain of transmission — using masks, drinking boiled water, eating hygienic food, controlling mosquitoes, using sterile needles, etc.
Q3. What are the principles of treatment of infectious diseases? Why is prevention better than cure?
Answer: Treatment of an infectious disease is based on two principles:
- Reducing the effects (symptoms) of disease: The patient is given complete rest to conserve energy. Medicines are given to bring down fever, reduce pain, control loose motions, reduce cough, etc. Adequate fluids and a nutritious diet help the body recover.
- Killing the cause of disease: Drugs are used that block essential biochemical processes of the microbe without harming our own cells. Antibiotics (e.g. penicillin) are used against bacteria; antiviral, antifungal, antimalarial and anthelmintic drugs are used against the respective microbes. The drug is chosen based on the type of microbe.
Why prevention is better than cure: (i) Once a person falls ill, even if cured, the body may suffer permanent damage — for example, polio cripples a child for life, and tuberculosis damages the lungs. (ii) The patient may pass the disease to many others before recovering, leading to an epidemic. (iii) Treatment takes time, money and physical suffering. (iv) Some diseases like AIDS have no cure. Therefore, by maintaining personal hygiene, drinking safe water, eating clean food, getting vaccinated and keeping the surroundings free of vectors, we can avoid disease altogether — which is better than curing it.
Q4. Explain the principle of vaccination/immunisation. Mention the role of the immune system in fighting diseases.
Answer: Principle of Vaccination: When a microbe enters the body for the first time, the immune system (mainly white blood cells) takes some time to recognise it and produce specific antibodies against it. During this time, the person falls ill. After the infection is over, the immune system “remembers” that microbe — this is called immune memory. If the same microbe enters again, the immune system attacks it very fast and the person does not fall ill. Vaccination uses this principle. A vaccine contains weakened, killed or part of a microbe; it cannot cause the actual disease but is enough to “fool” the immune system into making memory cells. So, when the real microbe enters later, the body is already prepared and destroys it before it can cause illness. This is called immunisation. Vaccines exist for tetanus, polio, measles, mumps, rubella, hepatitis B, BCG (tuberculosis), DPT (diphtheria, whooping cough, tetanus), etc.
Role of the Immune System: The immune system is the body’s natural defence against infections. White blood cells (lymphocytes, macrophages, etc.) recognise foreign microbes and destroy them, either by engulfing them or by producing antibodies. The immune system also creates memory cells for long-term protection. A person with a strong, well-nourished immune system rarely falls ill, while a weak immune system (as in AIDS or in malnourished children) makes a person prone to many infections. Good nutrition, exercise, sleep and hygiene help maintain a strong immune system.
Q5. Describe the symptoms, mode of spread and prevention of any three of the following diseases: TB, cholera, malaria, AIDS, dengue.
Answer:
- Tuberculosis (TB): Caused by the bacterium Mycobacterium tuberculosis. Symptoms — persistent cough for more than two weeks, fever especially in the evening, weight loss, blood in sputum, weakness. Spread — through air, when a TB patient coughs or sneezes. Prevention — BCG vaccination at birth, isolation of patients, covering mouth while coughing, good nutrition, full course of anti-TB drugs (DOTS).
- Cholera: Caused by Vibrio cholerae. Symptoms — severe watery diarrhoea (“rice-water stool”), vomiting, dehydration, muscle cramps. Spread — through contaminated water and food. Prevention — drink only boiled or safe water, eat freshly cooked hot food, wash hands with soap, proper sewage disposal, oral cholera vaccine in outbreaks, ORS to prevent dehydration.
- Malaria: Caused by the protozoan Plasmodium. Symptoms — high fever with shivering at regular intervals, sweating, headache, anaemia, enlarged spleen. Spread — bite of female Anopheles mosquito carrying the parasite. Prevention — use mosquito nets, repellents, full-sleeved clothes, remove stagnant water around houses, spray insecticides, use antimalarial drugs as advised.
- AIDS: Caused by HIV (Human Immunodeficiency Virus). Symptoms — weight loss, repeated infections, weakness; HIV destroys helper T-cells of the immune system. Spread — unprotected sexual contact, sharing of needles, infected blood transfusion, mother to baby. Prevention — safe sex, use of disposable sterile needles, screening of blood before transfusion, awareness; no vaccine yet.
- Dengue: Caused by the dengue virus. Symptoms — sudden high fever, severe headache, pain behind eyes, joint and muscle pain (“break-bone fever”), rash, in serious cases bleeding (dengue haemorrhagic fever). Spread — bite of female Aedes aegypti mosquito (bites in daytime). Prevention — destroy mosquito breeding spots (coolers, flower pots, tyres), use mosquito repellents, wear full-sleeved clothes, supportive treatment.
Additional MCQs
Q1. Which of the following is a viral disease?
(a) Tuberculosis (b) Cholera (c) AIDS (d) Typhoid
Answer: (c) AIDS
Q2. Malaria is caused by:
(a) Bacteria (b) Virus (c) Protozoan (d) Fungus
Answer: (c) Protozoan
Q3. Antibiotics are effective against:
(a) Viruses (b) Bacteria (c) Worms (d) All of these
Answer: (b) Bacteria
Q4. Which mosquito spreads dengue?
(a) Anopheles (b) Culex (c) Aedes (d) Mansonia
Answer: (c) Aedes
Q5. Which of the following is a chronic disease?
(a) Common cold (b) Tuberculosis (c) Influenza (d) Headache
Answer: (b) Tuberculosis
Q6. BCG vaccine is given against:
(a) Polio (b) Measles (c) Tuberculosis (d) Hepatitis
Answer: (c) Tuberculosis
Q7. Cholera spreads through:
(a) Air (b) Water (c) Vectors (d) Sexual contact
Answer: (b) Water
Q8. The full form of AIDS is:
(a) Acquired Immune Disease Syndrome (b) Acquired Immuno Deficiency Syndrome (c) Active Immune Disease Syndrome (d) Acute Immune Deficiency Syndrome
Answer: (b) Acquired Immuno Deficiency Syndrome
Q9. Which is a non-infectious disease?
(a) Cancer (b) Cholera (c) Malaria (d) Tuberculosis
Answer: (a) Cancer
Q10. Elephantiasis is caused by:
(a) Bacteria (b) Virus (c) Worm (d) Protozoan
Answer: (c) Worm (Wuchereria)
Fill in the Blanks
Q1. Diseases that last for a long time are called __________ diseases.
Answer: chronic
Q2. Tuberculosis is caused by a __________ called Mycobacterium.
Answer: bacterium
Q3. Malaria is spread by the female __________ mosquito.
Answer: Anopheles
Q4. The principle of vaccination is based on __________ memory.
Answer: immune
Q5. AIDS is caused by the __________ virus.
Answer: HIV
True or False
Q1. Antibiotics work against viruses.
Answer: False. Antibiotics work only against bacteria, not viruses.
Q2. Common cold is an acute disease.
Answer: True.
Q3. Cholera spreads through the bite of a mosquito.
Answer: False. Cholera spreads through contaminated water and food.
Q4. Personal health is independent of community health.
Answer: False. Personal health depends on community health, environment and public hygiene.
Q5. Vaccination prepares the immune system to fight specific microbes.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Health | State of complete physical, mental and social well-being. |
| Disease | Disturbance in the normal functioning of the body causing discomfort. |
| Symptom | An indication or sign of disease, like fever, cough, headache. |
| Acute Disease | Disease that lasts for a short time, e.g., common cold. |
| Chronic Disease | Disease that lasts for a long time, e.g., tuberculosis. |
| Infectious Disease | Disease caused by microbes and able to spread from one person to another. |
| Non-infectious Disease | Disease that does not spread; caused by genetic, lifestyle or deficiency factors. |
| Pathogen | Disease-causing microbe (bacteria, virus, fungus, protozoan, worm). |
| Vector | An animal (often insect) that carries microbes from one host to another. |
| Antibiotic | Drug that kills or stops the growth of bacteria, e.g., penicillin. |
| Vaccine | Preparation of weakened/killed microbes used to produce immunity. |
| Immunisation | Process of giving vaccine to make the body immune to a disease. |
| Immune System | Body’s defence system that fights infections (white blood cells, antibodies). |
| Antibody | Protein produced by the immune system to fight specific microbes. |
| Epidemic | Sudden spread of a disease to a large number of people in an area. |
| HIV | Human Immunodeficiency Virus — causes AIDS. |
| BCG | Vaccine given against tuberculosis. |
| ORS | Oral Rehydration Solution — used in cholera and diarrhoea to prevent dehydration. |
| DOTS | Directly Observed Treatment Short-course — treatment programme for TB. |
| Sanitation | Maintenance of cleanliness and proper waste disposal to prevent disease. |