Communicable Diseases (B3.1)
Pathogens
A pathogen is a microorganism that causes disease. Pathogens spread between individuals — these are communicable (infectious) diseases.
| Type | Example disease | How transmitted |
|---|---|---|
| Virus | Measles, HIV, influenza, TMV (tobacco mosaic virus) | Droplets, sexual contact, contaminated blood, direct contact |
| Bacterium | Salmonella, gonorrhoea | Food, sexual contact |
| Fungus | Athlete's foot, rose black spot | Spores via air or direct contact |
| Protist | Malaria | Vector (female Anopheles mosquito) |
How pathogens cause damage
- Viruses replicate inside host cells, destroying them (cell lysis) and triggering immune response.
- Bacteria produce toxins that damage tissues.
- Fungi produce spores; hyphae damage host tissues.
- Protists (Plasmodium — malaria): Anopheles mosquito injects Plasmodium into blood; parasites invade red blood cells, destroying them.
Key diseases to know
| Disease | Pathogen | Key facts |
|---|---|---|
| Measles | Virus | Droplet; fever, rash; can be fatal in malnourished children; prevented by MMR vaccine |
| HIV | Virus | Body fluids; attacks T-helper lymphocytes; progresses to AIDS if untreated; antiretrovirals slow progression |
| TMV | Virus | Direct plant contact; mosaic discolouration pattern on leaves; reduces photosynthesis |
| Salmonella | Bacterium | Food; vomiting, diarrhoea; poultry vaccinated in UK |
| Gonorrhoea | Bacterium | Sexual contact; thick yellow discharge; treated with antibiotics; antibiotic-resistant strains increasing |
| Malaria | Protist (Plasmodium) | Mosquito vector; fever, chills; prevented by nets/insecticides/antimalarial drugs |
| Rose black spot | Fungus | Airborne spores; black spots on leaves; reduced photosynthesis; treated with fungicide |
Body defences
Non-specific (physical/chemical barriers):
- Skin — barrier; sebum (oil) has antimicrobial properties
- Mucus and cilia in the airway — trap and sweep out pathogens
- Stomach acid — kills swallowed pathogens
Specific immune response:
- Phagocytosis — phagocytes engulf and digest pathogens
- Lymphocytes produce antibodies (specific to each antigen) and memory cells
- Memory cells → faster, larger response on re-infection (immunity)
Vaccination
Vaccine contains a harmless form of pathogen (dead/weakened/antigen). Immune system produces antibodies and memory cells. On re-exposure, memory cells produce antibodies rapidly — disease is prevented.
Herd immunity: if enough people are vaccinated, spread of disease is reduced even for unvaccinated individuals.
Antibiotics
Antibiotics kill bacteria (NOT viruses). Overuse leads to antibiotic resistance — bacteria with mutations survive, reproduce, and the resistant strain spreads. Strategies: complete courses, not prescribing for viral infections, agricultural controls.
Drug development
Stages: discovery → pre-clinical testing (cell cultures, animals) → clinical trials (Phase I — healthy volunteers; Phase II — small patient group; Phase III — large randomised double-blind placebo-controlled trial) → regulatory approval.
Double-blind: neither patient nor doctor knows who receives drug or placebo — prevents bias.
Common exam errors
- Saying antibiotics kill viruses — they kill bacteria only.
- Forgetting that vaccines work by producing memory cells (not just antibodies).
- Confusing vector (mosquito) with pathogen (Plasmodium) in malaria.
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