Pathogens are microorganisms that cause disease. There are four types:
Bacteria — single-celled organisms that reproduce rapidly. They produce toxins that damage cells and tissues. Examples: Salmonella (food poisoning), Gonorrhoea (STI).
Viruses — much smaller than bacteria, not living cells. They reproduce inside host cells and destroy them. Examples: Measles, HIV, Tobacco Mosaic Virus.
Fungi — some cause disease in plants and animals. Example: Rose black spot (plants).
Protists — single-celled eukaryotes. Example: Plasmodium (causes malaria, spread by mosquitoes).