ABSTRACT
The body is exposed to many pathogenic microorganisms and multicellular parasites. Most microorganisms found associated with the body are harmless and live as commensals or symbionts but others cause disease and are known as pathogens. Infection, from the Latin ‘inficere’ (to put in), is the successful persistence and/or multiplication of the pathogen on or within the host. In this respect, pathogenesis may be defined as the molecular and biochemical mechanisms that allow pathogens to cause diseases. Mummies preserved in ancient Egypt and elsewhere display evidence that infectious diseases have always been a threat. The emergence of new pathogens and the development of resistance to current treatments for existing pathogens means that infectious diseases will probably always be with us.
