Why Do We Fall Ill? – MCQs with Answers and Explanations
CBSE Class 9 — Science
Useful for: CBSE Board Exam (Class 9) | Format: 50 Topic-wise MCQs with Answers & Clear Explanations (NCERT-aligned)
This file contains 50 multiple choice questions (MCQs) organised topic-wise from NCERT Class 9 Biology Chapter Why Do We Fall Ill?. Each question includes the correct answer and an easy-to-understand explanation suitable for CBSE exam preparation and classroom tests. No countdown timer included — ready to paste into WordPress.
Topic: Types of Diseases & Causes (Questions 1–8)
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1. Which of the following is a non-infectious disease?A. Tuberculosis B. Diabetes C. Influenza D. ChickenpoxAnswer: B. Diabetes.Diabetes is non-infectious (metabolic) and cannot spread from person to person; others are infectious caused by microbes or viruses.
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2. Which microorganism is most likely responsible for causing ringworm?A. Bacteria B. Virus C. Fungus D. ProtozoanAnswer: C. Fungus.Ringworm is a fungal infection of skin (dermatophytes). It is not caused by bacteria or viruses.
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3. A disease that spreads from person to person is called:A. Hereditary disease B. Infectious disease C. Nutritional deficiency D. Congenital diseaseAnswer: B. Infectious disease.Infectious diseases are transmissible between individuals and are caused by pathogenic microorganisms.
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4. Which of the following causes malaria?A. Bacteria B. Virus C. Protozoan D. FungusAnswer: C. Protozoan.Malaria is caused by protozoan parasites of the genus Plasmodium, transmitted by Anopheles mosquitoes.
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5. A condition where the body's immune system attacks its own cells is called:A. Autoimmunity B. Allergy C. Infection D. ImmunodeficiencyAnswer: A. Autoimmunity.Autoimmune diseases arise when immune responses are directed against self-tissues (e.g., type 1 diabetes). This is not an infection or simple allergy.
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6. Which of these is an example of a hereditary non-infectious disease?A. Measles B. Scurvy C. Cystic fibrosis D. TuberculosisAnswer: C. Cystic fibrosis.Cystic fibrosis is a genetic disorder passed through inheritance; measles and TB are infectious, scurvy is nutritional deficiency.
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7. Which one of the following is NOT a microbial cause of disease?A. Virus B. Bacteria C. Worm D. FungiAnswer: C. Worm.Worms (helminths) are multicellular parasites, not microbes (microorganisms include bacteria, viruses, fungi, protozoa).
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8. Scurvy is caused due to deficiency of:A. Vitamin B B. Vitamin C C. Vitamin D D. IronAnswer: B. Vitamin C.Scurvy results from lack of vitamin C (ascorbic acid), leading to gum bleeding and poor wound healing.
Topic: Modes of Transmission (Questions 9–18)
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9. Which mode of transmission is most likely when people get infected by sharing the same drinking glass?A. Airborne B. Water-borne C. Fomite transmission D. Vector-borneAnswer: C. Fomite transmission.Fomites are objects (utensils, glasses) that can carry infectious agents and transmit disease when shared.
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10. Which disease is primarily transmitted by contaminated water?A. Typhoid B. Influenza C. Malaria D. RingwormAnswer: A. Typhoid.Typhoid is caused by Salmonella typhi and often spreads via faecal–oral route through contaminated water or food.
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11. Dengue fever is transmitted by:A. Aedes mosquito B. Anopheles mosquito C. Tsetse fly D. SandflyAnswer: A. Aedes mosquito.Dengue virus is transmitted by Aedes species (Aedes aegypti), which bite during daytime.
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12. Which practice helps prevent airborne transmission of respiratory infections?A. Boiling water B. Using mosquito nets C. Covering mouth while sneezing D. Avoiding raw vegetablesAnswer: C. Covering mouth while sneezing.Covering mouth and nose reduces dispersal of infectious droplets, limiting airborne/droplet spread.
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13. Faecal–oral transmission commonly spreads which of the following?A. Hepatitis A B. Tuberculosis C. Malaria D. PolioAnswer: A. Hepatitis A.Hepatitis A virus is typically transmitted by faecal–oral route via contaminated food and water.
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14. Which one is an example of vector-borne transmission?A. Typhoid by contaminated water B. Dengue by mosquito C. Measles by airborne droplets D. Athlete's foot by direct contactAnswer: B. Dengue by mosquito.Vector-borne diseases are spread by living carriers (vectors) like mosquitoes; dengue is a classic example.
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15. Which measure will NOT help prevent water-borne diseases?A. Boiling drinking water B. Using chlorine tablets C. Proper sewage disposal D. Covering food while cookingAnswer: D. Covering food while cooking.While covering food prevents contamination, it is not directly targeted at water-borne disease prevention; the other options directly make water safe.
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16. Which of the following can spread infection through droplets?A. Coughing B. Drinking contaminated water C. Mosquito bite D. Sharing clothesAnswer: A. Coughing.Coughing releases droplets that can contain infectious agents, transmitting respiratory infections.
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17. Food contamination by dirty hands is an example of:A. Airborne transmission B. Fomite transmission C. Vector transmission D. Vertical transmissionAnswer: B. Fomite transmission.Hands act as fomites transferring pathogens from faecal or respiratory sources to food, causing infection.
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18. Which action helps reduce vector breeding sites?A. Covering food B. Eliminating stagnant water C. Boiling water D. Regular handwashingAnswer: B. Eliminating stagnant water.Many vectors like mosquitoes breed in stagnant water; removing such habitats reduces vector populations and disease risk.
Topic: Immunity & Vaccination (Questions 19–28)
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19. Immunity obtained by vaccination is called:A. Natural active immunity B. Artificial active immunity C. Natural passive immunity D. Artificial passive immunityAnswer: B. Artificial active immunity.Vaccination stimulates the body's own immune system to produce memory cells—this is active immunity induced artificially.
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20. A newborn baby has temporary immunity because of antibodies received from the mother. This is:A. Artificial active immunity B. Natural passive immunity C. Artificial passive immunity D. Natural active immunityAnswer: B. Natural passive immunity.Antibodies passed through placenta or breast milk provide temporary natural passive immunity to the infant.
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21. Which cell type is primarily responsible for producing antibodies?A. Macrophages B. Red blood cells C. B lymphocytes (B cells) D. PlateletsAnswer: C. B lymphocytes (B cells).B cells differentiate into plasma cells that secrete specific antibodies against antigens.
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22. Herd immunity helps protect a population by:A. Reducing pathogen virulence B. Increasing disease symptoms C. Lowering transmission when most are immune D. Eliminating all pathogens from environmentAnswer: C. Lowering transmission when most are immune.When a high proportion is immune, spread is interrupted, indirectly protecting susceptible individuals.
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23. Which of the following is a live attenuated vaccine example?A. Toxoid vaccine B. Inactivated vaccine C. BCG vaccine (for tuberculosis) D. Subunit vaccineAnswer: C. BCG vaccine (for tuberculosis).BCG uses a live, attenuated strain of Mycobacterium bovis to stimulate immunity against TB.
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24. Which statement about vaccination is FALSE?A. Vaccination can prevent many infectious diseases. B. Vaccines provide immediate lifelong immunity without boosters. C. Vaccination contributes to herd immunity. D. Some vaccines require multiple doses for full protection.Answer: B. Vaccines provide immediate lifelong immunity without boosters.Not all vaccines provide lifelong immunity; some need boosters and take time to build immunity after administration.
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25. Which of these gives passive immunity artificially?A. Vaccine B. Antiserum injection C. Exposure to mild infection D. BreastfeedingAnswer: B. Antiserum injection.Antiserum contains pre-formed antibodies administered to provide immediate passive immunity (e.g., snake antivenom).
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26. After vaccination, the body forms:A. Memory cells B. Only toxins C. Only red blood cells D. Bacterial coloniesAnswer: A. Memory cells.Vaccination stimulates adaptive immune response producing memory B and T cells for faster response on re-exposure.
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27. Which organ is primarily involved in producing immune cells and is part of the lymphatic system?A. Heart B. Spleen C. Liver D. KidneyAnswer: B. Spleen.The spleen filters blood and helps produce and store immune cells; it's a key lymphoid organ.
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28. Which of the following best describes an antibody?A. A pathogen B. A molecule produced by immune system to neutralize antigens C. A type of white blood cell D. A vaccine ingredientAnswer: B. A molecule produced by immune system to neutralize antigens.Antibodies (immunoglobulins) are proteins secreted by plasma cells that bind specific antigens to neutralize or mark them for destruction.
Topic: Diagnosis, Treatment & Antibiotics (Questions 29–36)
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29. Which of the following is TRUE about antibiotics?A. They are effective against viruses. B. They kill or inhibit bacteria. C. They boost the immune system directly. D. They are a form of vaccine.Answer: B. They kill or inhibit bacteria.Antibiotics act against bacteria; they are not effective against viruses and are not vaccines.
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30. Overuse of antibiotics can lead to:A. Increased viral infections B. Antibiotic resistance C. Better nutrition D. Enhanced vaccine effectAnswer: B. Antibiotic resistance.Improper or excessive antibiotic use selects for resistant bacterial strains, making infections harder to treat.
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31. Which diagnostic test is commonly used for detecting malaria parasites in blood?A. Urine culture B. Blood smear microscopy C. X-ray D. Skin biopsyAnswer: B. Blood smear microscopy.Microscopic examination of stained blood smears reveals malaria parasites in infected red blood cells.
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32. Self-medication with leftover antibiotics is dangerous because:A. It ensures correct dosage B. It may be ineffective and contribute to resistance C. It acts as a vaccine D. It is harmlessAnswer: B. It may be ineffective and contribute to resistance.Using wrong antibiotics/dosages can fail to clear infection and promote resistant bacteria.
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33. Rapid diagnostic tests (RDTs) are useful because they:A. Require large labs B. Give quick results with minimal equipment C. Are always 100% accurate D. Replace all microscopyAnswer: B. Give quick results with minimal equipment.RDTs help in field or remote settings; while very helpful, they are not infallible and may need confirmation.
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34. Which is an appropriate first aid for a minor infected cut?A. Ignore it B. Clean with water and apply antiseptic C. Cover with dirty cloth D. Delay treatmentAnswer: B. Clean with water and apply antiseptic.Cleaning, applying antiseptic and covering protects wound and reduces infection risk; seek further care if severe.
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35. A bacterial infection producing pus is best indicated by:A. Pus formation and localized swelling B. Always high fever and rash C. Loss of appetite only D. Sneezing and runny noseAnswer: A. Pus formation and localized swelling.Pus indicates neutrophil activity against bacteria; localized swelling often accompanies bacterial infections.
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36. Which is a correct statement about antivirals?A. They kill bacteria B. They target viruses specifically C. They are the same as antibiotics D. They are used for fungal infectionsAnswer: B. They target viruses specifically.Antivirals are medicines designed to inhibit virus replication; they are different from antibiotics and antifungals.
Topic: Hygiene, Sanitation & Public Health (Questions 37–44)
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37. Which practice most effectively prevents faecal–oral disease transmission?A. Regular handwashing with soap B. Taking antibiotics daily C. Wearing masks outdoors D. Using mosquito repellentsAnswer: A. Regular handwashing with soap.Handwashing removes pathogens from hands, preventing transmission during eating or food preparation.
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38. Open defecation primarily increases risk of:A. Respiratory diseases B. Water-borne and diarrhoeal diseases C. Genetic disorders D. Vector-borne diseases onlyAnswer: B. Water-borne and diarrhoeal diseases.Open defecation contaminates environment and water, spreading faecal–oral pathogens causing diarrhoea and related illnesses.
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39. Which public health measure helps control spread during epidemics?A. Isolation of cases B. Encouraging mass gatherings C. Ignoring health reports D. Stopping vaccinationsAnswer: A. Isolation of cases.Isolating infected individuals reduces contact and transmission; other options worsen spread or are harmful.
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40. Which of these improves community sanitation the most?A. Building toilets and ensuring sewage disposal B. Drinking bottled water only C. Fencing open fields D. Wearing protective clothingAnswer: A. Building toilets and ensuring sewage disposal.Proper sanitation infrastructure prevents environmental contamination and reduces disease spread.
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41. Which of these behaviours contributes to antibiotic resistance at the community level?A. Completing prescribed antibiotic courses B. Buying antibiotics without prescription C. Using vaccination programs D. Practicing hygieneAnswer: B. Buying antibiotics without prescription.Unmonitored antibiotic use, incorrect doses and self-medication drive resistance; proper use prevents it.
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42. In outbreak investigation, identifying the source is important because:A. It helps target control measures B. It always cures patients C. It reduces need for hygiene D. It increases transmissionAnswer: A. It helps target control measures.Knowing the source (water, food, vector) allows specific interventions to stop further cases.
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43. Which of these is a primary prevention strategy?A. Vaccination B. Rehabilitation C. Surgery D. Physical therapyAnswer: A. Vaccination.Primary prevention aims to prevent disease before it occurs; vaccination is a key primary prevention measure.
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44. Which of the following is NOT a community-level action to prevent disease?A. Health education B. Ensuring clean water supply C. Encouraging open defecation D. Waste disposal managementAnswer: C. Encouraging open defecation.Open defecation increases disease risk; community actions should discourage it and promote sanitation.
Topic: Environmental & Occupational Health, Allergies (Questions 45–50)
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45. Air pollution primarily affects which body system?A. Digestive system B. Nervous system C. Respiratory system D. Reproductive systemAnswer: C. Respiratory system.Airborne pollutants irritate lungs and airways, worsening asthma and causing respiratory illnesses.
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46. Occupational exposure to certain chemicals can lead to:A. Increased immunity always B. Allergies or chronic diseases C. No health effects D. Instant vaccinationAnswer: B. Allergies or chronic diseases.Long-term exposure to toxins can cause occupational diseases, allergies, respiratory or systemic illnesses.
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47. An allergic reaction is caused by:A. Immune system overreaction to harmless substances B. Bacterial infection only C. Viral infection only D. Lack of sleepAnswer: A. Immune system overreaction to harmless substances.Allergies occur when the immune system mistakenly reacts to allergens (pollen, food, dust) causing symptoms.
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48. Which of the following reduces exposure to industrial air pollutants in a community?A. Planting trees and enforcing emission limits B. Building more factories without controls C. Encouraging burning of waste openly D. Removing green spacesAnswer: A. Planting trees and enforcing emission limits.Tree planting and regulations reduce pollutant levels; other options worsen air quality.
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49. Which symptom suggests an immediate severe allergic reaction (anaphylaxis)?A. Mild itching only B. Rapid breathing and difficulty in breathing C. Slight cough D. Sneezing onceAnswer: B. Rapid breathing and difficulty in breathing.Anaphylaxis causes airway constriction and circulatory collapse—difficulty breathing requires urgent medical care.
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50. Which is an effective personal measure to lower risk of infections?A. Frequent handwashing B. Sharing personal items C. Avoiding vaccination D. Not treating woundsAnswer: A. Frequent handwashing.Hand hygiene is a simple yet highly effective method to prevent transmission of many infectious agents.