Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air.
Bacteria may reproduce rapidly inside the body and may produce poisons (toxins) that damage tissues and make us feel ill.
Viruses may reproduce rapidly inside the body. They live and reproduce inside cells, causing cell damage
