Diseases

1. Bacteria.

• Tiny single-celled organisms.
• Reproduce rapidly (every 20 mins in good conditions).
• Cause disease by RELEASING TOXINS or by DAMAGING TISSUES directly.
• Example diseases: cholera (gut), tuberculosis (lungs), tetanus (toxin), salmonella (food poisoning), strep throat.

2. Viruses.

• NOT cells — protein coat + nucleic acid (DNA/RNA).
• ~100× smaller than bacteria.
• Hijack host cells to reproduce — burst out and infect more cells.
• Cannot live independently; need a host.
• Examples: flu, common cold, HIV, COVID-19, measles, mumps, rabies.

3. Fungi.

• Multicellular (or single-celled like yeasts).
• Often grow on/in skin or other warm moist places.
• Cause itching, scaling, infection.
• Examples: athlete's foot, ringworm, thrush, nail fungus.

4. Protoctists.

• Single-celled eukaryotes.
• Often need a vector to enter the body.
• Examples: malaria (caused by Plasmodium), amoebic dysentery.

Worked qualitative. Why don't antibiotics work against viruses?

• Antibiotics target processes UNIQUE to bacterial cells (like cell-wall synthesis).
• Viruses don't have these — they use the host's machinery.
• Antiviral drugs are different — they target viral-specific steps (e.g. reverse transcriptase for HIV).
• Taking antibiotics for a viral infection (cold, flu) is useless and contributes to resistance.

Cambridge tip. Always classify the pathogen by its FOUR-type group. 'Microbe' or 'germ' is too vague.

1. Direct contact.

• Pathogen passes when people TOUCH, kiss, have sex.
• Examples: HIV (sex), athlete's foot (shared towels), herpes (kissing/sex).
• Prevention: hygiene, condoms, not sharing personal items.

2. Air (droplets).

• Coughs and sneezes shoot infected droplets.
• Inhaled by others nearby.
• Examples: flu, common cold, COVID-19, TB.
• Prevention: masks, ventilation, distancing, covering mouth when coughing.

3. Water and food.

• Pathogens enter water/food via faeces, contaminated hands, dirty surfaces.
• Drinking/eating → enters gut.
• Examples: cholera, typhoid, salmonella, E. coli food poisoning.
• Prevention: clean water, sewage treatment, washing hands, cooking food properly.

4. Vectors.

• Animal carries the pathogen between hosts without being affected.
• Examples:
  • Mosquito (Anopheles) → malaria.
  • Mosquito (Aedes) → dengue, Zika.
  • Tick → Lyme disease.
  • Dog (saliva) → rabies.
• Prevention: insecticides, bed nets, draining stagnant water, vaccination of animals.

Worked qualitative. Why was clean drinking water in 19th-century London so important?

• Cholera was rampant in slums where sewage and drinking water mixed.
• John Snow traced an outbreak to one contaminated pump (1854).
• Clean water and sewers cut cholera deaths by ~95%.
• Sanitation infrastructure remains the single biggest health intervention in history.

Cambridge tip. Cambridge often gives a scenario and asks students to identify the route. Read carefully — 'sneezed near someone' = air; 'shared a needle' = direct/blood; 'mosquito bite' = vector.

Before the immune system kicks in, the body has a series of physical and chemical barriers.

1. Skin (physical).

• Outer layer: dead, keratin-rich cells. Tough.
• Continuously sheds and replaces.
• Sweat is slightly acidic + contains some antimicrobial chemicals.
• Damaged skin (cuts, burns) breaks the barrier — wounds heal fast to restore it.

2. Mucus and cilia (airways).

• GOBLET cells in the airway lining secrete sticky MUCUS.
• Mucus traps pathogens and dust.
• CILIATED cells (with tiny hair-like extensions) BEAT in waves to sweep mucus UP towards the throat.
• Mucus is then swallowed (acid kills pathogens) or coughed out.
• Smoking damages cilia → mucus pools → 'smoker's cough'.

3. Stomach acid (chemical).

• HCl pH ~2.
• Kills most bacteria swallowed with food/drink.
• First line of defence against gut infections.

4. Tears, saliva, sweat (lysozyme).

• Contain LYSOZYME — an enzyme that breaks down bacterial cell walls.
• Eyes, mouth and skin surface get continuous antimicrobial coverage.

5. Stomach + gut barrier in addition to acid:

• Mucus lining of gut = mechanical barrier.
• Helpful gut bacteria compete with pathogens.

Worked qualitative. Why does smoking cause more lung infections?

• Tobacco smoke paralyses or kills cilia.
• Mucus + trapped pathogens accumulate in the lungs.
• More chance for pathogens to multiply → bronchitis, pneumonia.
• Heavy smokers: chronic 'smoker's cough' is the body trying to clear stagnant mucus.

Cambridge tip. Cambridge marks the MECHANISM, not just the part. Don't say 'tears clean eyes' — say 'tears contain lysozyme which breaks bacterial cell walls'.