Why Do We Fall Ill — Class 9 Biological Science
1. What Is Health?
Health is a state of complete PHYSICAL, MENTAL, and SOCIAL well-being — not merely the ABSENCE of disease. (WHO definition).
Conditions for Good Health
- Nutrition: Balanced diet.
- Clean environment: Clean drinking water, sanitation, fresh air.
- Exercise: Physical activity.
- Rest and sleep: Adequate.
- Mental well-being: Freedom from stress, social harmony.
2. Disease — When Things Go Wrong
A DISEASE is a condition where the normal functioning of the body or any of its parts is DISTURBED. Symptoms: Observable signs that something is wrong (fever, pain, rash, cough). Signs: Measurable indicators (temperature, blood pressure, pulse rate).
Acute vs Chronic Diseases
| Feature | Acute | Chronic |
|---|---|---|
| Duration | SHORT (days to weeks) | LONG (months to years, sometimes life-long) |
| Onset | SUDDEN | GRADUAL |
| Long-term effects | Usually recover fully | May cause permanent damage |
| Examples | Common cold, flu, diarrhoea | Diabetes, tuberculosis, asthma, arthritis |
3. Causes of Disease
Infectious (Communicable) Diseases
Caused by PATHOGENS that can spread from one person to another.
| Pathogen Type | Diseases |
|---|---|
| Virus | Common cold, influenza, dengue, chikungunya, measles, mumps, AIDS (HIV), COVID-19 |
| Bacteria | Typhoid, cholera, tuberculosis (TB), tetanus, diphtheria, plague |
| Protozoa | Malaria (Plasmodium), kala-azar (Leishmania), amoebic dysentery |
| Fungi | Ringworm, athlete's foot |
| Multicellular (worms) | Filariasis (elephantiasis), tapeworm, roundworm |
Non-Infectious (Non-Communicable) Diseases
NOT caused by pathogens. Do NOT spread from person to person. Examples: Diabetes, hypertension (high BP), heart disease, cancer, asthma, arthritis. Causes: Genetic, lifestyle (diet, exercise, smoking, alcohol), environmental factors.
4. Means of Spread of Infectious Diseases
| Route | How It Spreads | Examples |
|---|---|---|
| Air | Droplets from sneeze/cough | Common cold, flu, TB, COVID-19 |
| Water/Food | Contaminated water/food with pathogen | Cholera, typhoid, hepatitis A, polio |
| Physical contact | Direct touch. Sexual contact. | Ringworm, AIDS, syphilis |
| Vectors | Insects carry pathogen from host to host | Malaria (female Anopheles mosquito), dengue (Aedes aegypti), plague (rat flea) |
| Blood-to-blood | Infected needles, transfusion | AIDS, hepatitis B |
5. Principles of Treatment
Symptom-based: Reduce symptoms (fever → paracetamol, pain → analgesic). This does NOT kill the pathogen. Pathogen-based: Kill the pathogen. Antibiotics (for bacteria — e.g., penicillin, streptomycin). Antivirals. Antifungals.
Why Antibiotics Don't Work on Viruses
'Antibiotics block bacterial pathways (like cell wall synthesis) that viruses DON'T have. Viruses use the HOST'S machinery to multiply — killing the virus without killing the host cell is very difficult. This is why anti-viral drugs are fewer and more complex.'
6. Principles of Prevention
General Prevention
- Sanitation: Clean water, proper sewage disposal, garbage management.
- Nutrition: Balanced diet strengthens the IMMUNE SYSTEM.
- Hygiene: Hand washing, personal cleanliness.
- Vector control: Mosquito nets, spraying insecticides, not allowing water stagnation.
- Isolation: Keeping infected persons separate during infectious period.
Specific Prevention — Vaccination (Immunisation)
Vaccines introduce WEAKENED or DEAD pathogens (or parts of them) into the body. The immune system produces ANTIBODIES against them. Memory cells REMEMBER the pathogen. On subsequent exposure, the immune response is RAPID and STRONG — the person does NOT fall ill. 'Vaccination is one of the greatest public health achievements in human history. Smallpox has been ERADICATED. Polio is nearly eradicated. These victories were won through vaccination.'
Common Vaccines
| Vaccine | Disease Prevented |
|---|---|
| BCG | Tuberculosis |
| Oral Polio Vaccine (OPV) | Polio |
| DPT | Diphtheria, Pertussis (whooping cough), Tetanus |
| MMR | Measles, Mumps, Rubella |
| Hepatitis B | Hepatitis B |
| COVID-19 vaccines | COVID-19 |
7. The Immune System
The body's DEFENCE system. Innate immunity: Present from birth — skin, mucus, stomach acid, WBCs. Acquired immunity: Developed after exposure to a specific pathogen — antibodies and memory cells. 'Chickenpox usually occurs only ONCE in a lifetime. After the first infection, the immune system REMEMBERS the virus — subsequent exposures are neutralised before symptoms develop.'
8. AP Public Health Context
- Andhra Pradesh has made significant progress in institutional deliveries and maternal health through schemes like YSR Aarogya Sri (health insurance for BPL families).
- Vector-borne diseases: Coastal AP districts (East Godavari, Visakhapatnam, Nellore) are vulnerable to malaria and dengue — seasonal outbreaks require coordinated public health responses.
- Sanitation: Swachh Bharat Mission has improved toilet coverage in rural AP, reducing water-borne disease incidence.
9. Common Mistakes
- 'Antibiotics cure all infections': Antibiotics work ONLY against BACTERIA. They are useless against viruses, fungi, and protozoa.
- 'A person with no symptoms is healthy': Some diseases (like hypertension, early HIV, early TB) have NO symptoms initially. Health is more than absence of symptoms.
- 'All diseases spread by contact': Some are air-borne, water-borne, or vector-borne. Mode of transmission determines prevention.
10. AP Exam Focus
| Topic | Marks |
|---|---|
| Acute vs Chronic | 2-3 |
| Infectious vs Non-infectious | 3-4 |
| Means of spread | 3-4 |
| Principles of treatment and prevention | 4-5 |
Quick Self-Test
- Define health (WHO). (Answer: State of complete physical, mental, and social well-being.)
- Difference between acute and chronic disease? (Answer: Acute: short duration, sudden onset. Chronic: long duration, gradual onset.)
- Why don't antibiotics work on viruses? (Answer: Viruses use host cell machinery. Antibiotics target bacterial pathways viruses don't have.)
- Name two vector-borne diseases and their vectors. (Answer: Malaria — Anopheles mosquito. Dengue — Aedes mosquito.)
- What is vaccination? (Answer: Introducing weakened/dead pathogens to stimulate antibody production and memory cells.)
