A pathogen is a microorganism that causes an infectious disease. Diseases caused by pathogens are communicable: they pass from one person (or plant) to another. Heart disease or scurvy are non-communicable: nothing is caught.

• Bacteria reproduce quickly and may release toxins that damage tissues.
• Viruses are far smaller. They live and reproduce inside cells, damaging them.
• Fungi and protists cause disease in plants and animals too.

Pathogens spread by air (droplets from coughs and sneezes), by water, by direct contact, and through vectors such as mosquitoes.

Disease	Pathogen	Spread	Control
measles	virus	droplets	vaccinate children
HIV	virus	sex, shared needles	condoms, clean needles; antiretroviral drugs
tobacco mosaic virus	virus	plant contact, tools	destroy infected plants
salmonella	bacterium	undercooked food	vaccinate poultry, cook well
gonorrhoea	bacterium	unprotected sex	condoms, antibiotics
rose black spot	fungus	spores in wind and water	remove leaves, fungicide
malaria	protist	mosquito bites	nets, stop mosquitoes breeding

Edexcel and OCR also use cholera (water), tuberculosis (droplets), chlamydia and ash dieback.

Viruses are not killed by antibiotics. Salmonella and gonorrhoea are bacteria; measles and HIV are viruses.

Each division turns one bacterium into two, so the number doubles every division.

number of bacteria = starting number × 2ⁿ (n = number of divisions)
n = time ÷ mean division time

1. 20 bacteria, dividing every 30 minutes, for 4 hours
2. 4 × 60 = 240 minutes; 240 ÷ 30 = 8 divisions
3. 20 × 2⁸ = 20 × 256 = 5120 = 5.12 × 10³

Multiply by 2 once for every division: 2⁸, not 2 × 8.