Why Do We Fall Ill? – Long Answer Type Questions
Class: 9
Subject: Science — Biology
Chapter: 14 — Why Do We Fall Ill?
30 Long Answer Questions (Model Answers)
NCERT-aligned long answer questions with structured model answers to help Class 9 students write exam-ready responses.
Content Bank Snapshot:
Topics included: definition of disease, infectious vs non-infectious diseases, major pathogens (bacteria, viruses, fungi, protozoa, helminths), modes of transmission, symptoms and diagnosis, immunity (innate & adaptive), vaccination principles, antibiotics and resistance, public health measures, prevention and control strategies.
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1. Define disease and explain its impact on the body.A disease is any condition that impairs the normal structure or function of an organism, producing signs and symptoms. Diseases disrupt homeostasis — for example, infections may damage tissues, trigger inflammation, reduce organ efficiency and cause systemic effects like fever or fatigue. Psychologically and socially, disease can reduce quality of life and productivity. Effective diagnosis and treatment aim to restore normal function and prevent complications.
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2. Distinguish between infectious and non-infectious diseases with examples.Infectious diseases are caused by living pathogens (bacteria, viruses, fungi, protozoa, helminths) and can spread between hosts; examples include malaria and tuberculosis. Non-infectious diseases are not contagious and include genetic disorders, nutritional deficiencies and lifestyle illnesses such as diabetes and cardiovascular disease. Their causes, prevention and treatment strategies differ: infectious diseases often require public health measures and antimicrobials, while non-infectious diseases focus on lifestyle modifications and long-term management.
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3. Describe the structure and general characteristics of bacteria and give two examples of bacterial diseases.Bacteria are unicellular prokaryotic organisms lacking a membrane-bound nucleus; their genetic material is in a nucleoid. Many have a rigid cell wall, flagella for movement and reproduce by binary fission. Some are free-living and beneficial (soil bacteria), while pathogenic species cause disease. Examples: Mycobacterium tuberculosis causes tuberculosis; Vibrio cholerae causes cholera. Bacteria can be cultured on media and are treated with antibiotics when appropriate.
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4. Explain why antibiotics are effective against bacteria but not viruses. Include the concept of antibiotic resistance.Antibiotics target bacterial-specific structures or processes such as cell wall synthesis, protein synthesis or DNA replication. Viruses lack these structures—they are acellular and replicate inside host cells using host machinery—so antibiotics have no effect on them. Misuse or overuse of antibiotics promotes selection of resistant bacterial strains that survive treatment. Antibiotic resistance occurs via mutation or horizontal gene transfer, making infections harder to treat and requiring stewardship: correct prescription, full courses and avoiding unnecessary use.
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5. Describe the general structure of a virus and briefly explain how viruses reproduce.Viruses are simple infectious agents composed of nucleic acid (DNA or RNA) enclosed in a protein coat called a capsid; some have a lipid envelope. They cannot carry out metabolism or reproduce independently and must infect host cells. After entry, viral genomes hijack host cell machinery to replicate viral components, assemble new virions and release them, often damaging or killing the host cell. Antiviral strategies target various stages of this cycle or aim to prevent infection through vaccination.
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6. Explain fungal infections: give examples, modes of spread and basic treatment approaches.Fungal infections are caused by eukaryotic fungi; common types include superficial infections like ringworm and athlete’s foot, and systemic infections in immunocompromised individuals. They spread by direct contact or via spores in the environment. Diagnosis uses microscopy or culture. Treatment includes topical or systemic antifungal drugs depending on severity, along with hygiene measures to prevent spread. Environmental control and avoiding prolonged moisture on skin help reduce incidence.
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7. Describe protozoan diseases with emphasis on malaria: causative organism, life cycle steps (brief), symptoms and prevention.Protozoa are single-celled eukaryotic parasites; malaria is caused by Plasmodium species transmitted by Anopheles mosquitoes. The parasite has stages in human liver and red blood cells and sexual stages in mosquitoes. Symptoms include cyclic fever, chills, anaemia and fatigue. Prevention focuses on vector control (mosquito nets, insecticides), prompt diagnosis and antimalarial treatment. Public health measures to reduce mosquito breeding are crucial to control transmission.
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8. What are helminthic infections? Discuss transmission and control measures for intestinal worms.Helminthic infections are caused by parasitic worms (roundworms, tapeworms, flukes). Transmission often occurs via contaminated food, water or soil (egg ingestion) or undercooked meat. Control includes sanitation, deworming programmes, health education, safe food practices and wastewater management. Individual treatment uses specific anthelmintic drugs; community-level measures reduce reinfection risk.
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9. Define 'mode of transmission' and describe four common modes with examples.Mode of transmission describes how pathogens move from a source to a susceptible host. Four common modes: (1) Direct contact—touching or sexual contact (e.g., scabies); (2) Droplet/airborne—sneeze or cough droplets carrying pathogens (influenza, TB); (3) Vehicle-borne—contaminated food or water (cholera, typhoid); (4) Vector-borne—transmission by animals like mosquitoes (malaria, dengue). Understanding modes guides prevention strategies.
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10. Explain the terms 'symptom' and 'sign' and provide two examples of each for infectious diseases.A symptom is a subjective experience reported by the patient (e.g., pain, fatigue), while a sign is an objective finding observable by others (e.g., fever measured by thermometer, rash). For respiratory infections: symptom—sore throat; sign—elevated body temperature. For cholera: symptom—severe thirst; sign—profuse watery diarrhoea observed clinically.
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11. Describe the first-line physical and chemical barriers of the human body that prevent infections.First-line barriers include skin (physical barrier), mucous membranes that trap microbes, ciliated epithelium in the respiratory tract that moves pathogens out, acidic stomach environment that destroys ingested microbes, and secretions like tears and saliva containing antimicrobial enzymes (lysozyme). These nonspecific defenses reduce pathogen entry and are part of innate immunity, acting immediately to prevent infection establishment.
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12. Explain innate immunity (non-specific) and give two cellular mechanisms involved.Innate immunity is the immediate, non-specific defence that responds to invaders irrespective of prior exposure. Key cellular mechanisms include phagocytosis by macrophages and neutrophils that engulf pathogens, and natural killer (NK) cells that destroy infected or abnormal cells. Innate responses also include inflammation and fever which limit pathogen spread and recruit immune cells to the infection site.
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13. Describe adaptive (specific) immunity and contrast humoral and cell-mediated responses.Adaptive immunity develops after exposure to specific antigens and generates memory for faster secondary responses. Humoral immunity involves B-lymphocytes that differentiate into plasma cells producing antibodies which neutralise pathogens or mark them for destruction. Cell-mediated immunity involves T-lymphocytes: helper T-cells activate other immune cells, while cytotoxic T-cells kill infected host cells. Both arms coordinate to clear infections and establish immune memory.
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14. Explain how vaccination works and why it is important for public health.Vaccination exposes the immune system to a harmless form or component of a pathogen (attenuated, inactivated or subunit), stimulating an adaptive response and formation of memory B and T cells without causing disease. This enables rapid, effective responses upon future exposure. Widespread vaccination reduces disease incidence, prevents outbreaks and can achieve herd immunity which protects those who cannot be vaccinated, making vaccination a cornerstone of public health programmes.
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15. Describe different types of vaccines (brief) with one example each.Vaccine types include: live attenuated (weakened pathogen; e.g., measles vaccine), inactivated/killed (e.g., inactivated polio vaccine), subunit/conjugate vaccines containing specific antigens (e.g., Hepatitis B), and toxoid vaccines using inactivated toxins (e.g., tetanus). Each type has advantages and considerations for safety, duration of immunity and storage.
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16. Discuss the concept of herd immunity and its limitations.Herd immunity occurs when a sufficient proportion of a population is immune, reducing pathogen transmission and indirectly protecting susceptible individuals. Its effectiveness depends on vaccine coverage and pathogen transmissibility; highly contagious diseases require very high coverage. Limitations include pockets of unvaccinated individuals, waning immunity over time, and variants that escape immunity. Herd immunity complements but does not replace individual vaccination and public health measures.
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17. Explain how public health measures control waterborne diseases like cholera.Control of waterborne diseases involves ensuring clean drinking water through filtration, chlorination, sewage treatment and safe disposal of waste to reduce contamination. Health education on boiling water, hand hygiene, and safe food handling reduces exposure. During outbreaks, prompt rehydration therapy and antibiotics for severe cases, along with surveillance and infrastructure improvements, control spread and prevent recurrence.
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18. Describe the importance of sanitation and hygiene in preventing infectious diseases.Sanitation (safe waste disposal, sewage systems) prevents environmental contamination by pathogens, reducing waterborne and vector-borne diseases. Hygiene practices such as handwashing, safe food preparation and personal cleanliness interrupt transmission routes. Together with clean water supply and proper waste management, these measures form the basis of community-level disease prevention and improve overall public health.
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19. Discuss diagnostic methods used to identify infectious agents in the laboratory.Diagnostic methods include microscopic examination of clinical specimens, culture of bacteria or fungi on selective media, serological tests to detect antibodies or antigens, and molecular techniques like PCR to detect pathogen nucleic acids. Imaging and biochemical tests may also aid diagnosis. Accurate laboratory diagnosis guides targeted treatment and epidemiological control measures.
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20. Explain how antibiotic sensitivity testing guides treatment.Antibiotic sensitivity testing (e.g., disc diffusion) determines which antibiotics effectively inhibit a bacterial isolate. Results help clinicians choose effective drugs, avoid ineffective treatments and reduce the development of resistance. Sensitivity testing is especially important for serious infections and when resistance is suspected, enabling personalised and evidence-based therapy.
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21. Describe three strategies to control vector-borne diseases at community level.Strategies include: eliminating mosquito breeding sites by removing stagnant water and covering containers; using insecticide-treated bed nets and indoor residual spraying to reduce human-vector contact; and community education campaigns to promote protective behaviours. Integrated vector management combining environmental, biological and chemical controls is most effective and sustainable.
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22. Explain the role of nutrition in disease prevention and immune function.Adequate nutrition provides the vitamins, minerals and macronutrients required for optimal immune function. Deficiencies (e.g., vitamin A, zinc, protein-energy malnutrition) impair immune responses, increase susceptibility to infection and worsen outcomes. Good nutrition supports barrier integrity, antibody production and cellular immunity, making it an essential component of disease prevention and recovery.
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23. Discuss the interplay between host, agent and environment in the epidemiology of infectious diseases.Disease occurrence depends on interactions among the host (immunity, behaviour), the agent (virulence, infectivity) and the environment (sanitation, climate, vector presence). For example, malaria requires a susceptible human, infectious Plasmodium and an environment with Anopheles mosquitoes and suitable breeding sites. Public health interventions target one or more of these factors to break transmission chains and reduce disease burden.
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24. Explain isolation and quarantine — differences and purposes in outbreak control.Isolation separates infected individuals to prevent them infecting others; quarantine restricts movement of exposed but asymptomatic persons to see if they develop disease. Isolation is used for confirmed cases, while quarantine is preventive for contacts. Both reduce transmission during outbreaks and are important tools alongside testing and contact tracing.
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25. Describe how surveillance systems help in early detection and control of epidemics.Surveillance collects and analyses health data to detect unusual increases in disease incidence. Early warning systems enable rapid investigation, containment measures, resource allocation and public communication. Routine reporting from clinics, laboratories and sentinel sites forms the backbone of surveillance, allowing authorities to respond before outbreaks spread widely.
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26. Discuss the role of education and behaviour change communication in preventing disease.Education informs people about transmission, prevention and when to seek care; behaviour change communication uses targeted messages and community engagement to modify risky behaviours (e.g., open defecation, poor hand hygiene). Sustained education increases uptake of vaccination, improves sanitation practices and empowers communities to adopt healthier behaviours, thereby reducing disease transmission and improving health outcomes.
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27. Explain principles of rational antibiotic use and stewardship.Rational antibiotic use involves prescribing antibiotics only when bacterial infection is confirmed or highly suspected, choosing appropriate agents and durations guided by sensitivity tests, and avoiding antibiotics for viral infections. Stewardship programmes monitor and optimise antibiotic prescribing, educate clinicians and the public, and implement guidelines to slow resistance development and preserve antibiotic efficacy.
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28. Describe the diagnostic and treatment challenges posed by emerging and re-emerging infectious diseases.Emerging diseases may be caused by novel pathogens for which diagnostics, treatments and vaccines are lacking. Re-emerging diseases may show drug resistance or return due to changed ecology. Challenges include limited surveillance, rapid spread, diagnostic delays, lack of specific therapies and the need for rapid research. Global coordination, investment in diagnostics, and flexible public health systems are required to manage these threats.
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29. Provide an exam-ready answer explaining how vaccination, sanitation and antibiotics together reduce disease burden in a community.Vaccination prevents susceptible individuals from becoming infected, reducing transmission and protecting the population. Sanitation and clean water reduce environmental sources of pathogens, lowering exposure. Antibiotics treat bacterial infections effectively when used properly, reducing morbidity and mortality. Together, they form prevention, environmental control and medical treatment pillars that synergise to reduce disease burden and improve community health.
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30. As a student, outline a concise study-plan to master this chapter and score well in board examinations.Study the NCERT text thoroughly and memorise essential definitions and examples. Make comparative tables (infectious vs non-infectious, pathogens and their diseases), practise labelled diagrams (e.g., life cycle of malaria parasite), and write model long answers using the define-explain-example-prevention structure. Solve previous years’ questions, do timed practice, revise regularly and clear doubts with teachers. Include current examples (vaccination campaigns, outbreak control) to enrich answers and demonstrate application knowledge.