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 micro-organisms 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.