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Disease

Pathogens, transmission, immunity, vaccination

Pathogens — Infectious Agents

Bacteria: Prokaryotes (~1-5 μm). Cause: cholera, typhoid, gonorrhoea. Antibiotics kill them (target cell wall/ribosomes).

Viruses: Non-living (0.02-0.2 μm). Cause: flu, measles, HIV. Replicate inside host cells. Antibiotics don't work.

Fungi: Cause: athlete's foot, ringworm. Antifungal creams treat.

Protoctists: Cause: malaria (Plasmodium via mosquito). Antimalarial drugs treat.

Disease Transmission — How Diseases Spread

Droplet infection: Coughing/sneezing sprays pathogen (e.g., flu, measles). Distance: ~2 m.

Direct contact: Touching infected skin (e.g., ringworm, sexually transmitted infections).

Contaminated food/water: Pathogens in unsterile water (e.g., cholera). Cooking kills most pathogens.

Vectors: Animals carrying pathogens (e.g., mosquitoes → malaria, ticks → Lyme disease).

Blood transmission: Contaminated needles (e.g., HIV, hepatitis B).

First Line of Defence — Non-Specific

Skin: Physical barrier. Dead cells shed pathogens. Sebum (oily) is antibacterial.

Mucus: Lining nose/trachea/gut. Traps pathogens. Cilia sweep them out.

Stomach acid (pH 2): Kills most ingested pathogens.

Lysozyme: Enzyme in tears, saliva, mucus. Breaks down bacterial cell walls.

Second Line of Defence — Specific Immune Response

Phagocytes (WBC): Engulf pathogens (phagocytosis). Include macrophages, neutrophils.

Lymphocytes (WBC): Produce antibodies specific to pathogens.

Antibodies: Proteins made by B-lymphocytes. Bind to antigens on pathogen. Neutralise/tag for destruction.

Memory cells: Remain after infection. Produce fast antibody response if same pathogen returns.

Active Immunity vs Passive Immunity

Type	How Acquired	Response Speed	Duration	Memory Cells
Active	Infection or vaccination	Slow (1-2 weeks)	Long-lasting	Yes
Passive	Antibodies from blood/mother	Immediate	Short (weeks)	No

Active: Your own immune system makes antibodies — long-term protection
Passive: Borrow antibodies from outside — temporary protection

Vaccination Mechanism

Vaccine contains: Dead/weakened pathogen or antigen (safe, non-infectious)

Process: Immune system recognises antigen → B-lymphocytes make antibodies → memory cells form

If real pathogen later: Memory cells rapidly produce antibodies → fast response → don't get sick (or mild illness)

Examples: Polio (oral vaccine, weakened), MMR (weakened viruses), COVID (mRNA teaches body to make spike protein)

Cholera Case Study

Pathogen: Vibrio cholerae (bacterium)

Transmission: Contaminated water in poor sanitation areas

Mechanism: Bacteria → produce toxin → toxin damages intestinal lining → uncontrolled water loss → diarrhoea → severe dehydration → death

Prevention: Clean water, sanitation, vaccination, oral rehydration therapy (ORS), antibiotics

Kills thousands in low-income countries; rare in developed countries

Exam Tips

✓ Know 4 pathogen types: bacteria, virus, fungi, protoctists

✓ Transmission routes: droplet, contact, food/water, vector

✓ First defence: skin, mucus, acid, lysozyme (non-specific)

✓ Active immunity: slow, long-lasting, memory cells

✓ Vaccination: dead/weakened pathogen → memory cells form

Key Takeaways

✅ Pathogens: bacteria, virus, fungi, protoctists

✅ Transmission: droplet, contact, food/water, vectors, blood

✅ First defence: skin, mucus, acid (non-specific)

✅ Immune response: phagocytes + lymphocytes (antibodies)

✅ Active immunity: slow but long-lasting; passive: fast but short

✅ Vaccination: primes immune system with memory cells

Your immune system is your shield! 🦠✓