Many microorganisms are capable of causing infections and illness in people. These include bacteria, viruses, fungi and protozoa.

Some of these microorganisms represent a particular threat to the community because they are easily spread from one person to another and they can cause serious and sometimes fatal infections.

Some microorganisms are dangerous because many people may be infected from the same environmental source. For example, many people can be affected through the ingestion of a contaminated food item.

Other serious microorganisms which are actually more difficult to transmit might still cause high levels of community concern and social disruption just by their reputation.