Micro-organisms that cause diseases, parasitising plants, animals and humans (see PARASITE). Some organisms are frequently PATHOGENIC, whereas others rarely cause disease. Opportunistic pathogens are those which rarely cause serious infection in healthy people but can do so in patients with weakened immune systems (immunocompromised – see IMMUNITY). Pathogens include BACTERIA, viruses (see VIRUS), prions (see PRION), fungi (see FUNGUS), PROTOZOA and metazoa (multicellular microorganisms called HELMINTHS or worms). The pathogenicity of an organism is called its virulence, which is measured by the number of organisms required to cause disease. The 50 per cent of lethal dose (LD50) is the quantity of a particular pathogen needed to cause infection in half of the hosts invaded.... pathogens
adj. capable of causing disease. The term is applied to a parasitic microorganism (especially a bacterium) in relation to its host. —pathogenicity n.... pathogenic
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