(Science: epidemiology) Typically, viruses, bacteria, fungi and protozoa. More generally, parasites that multiply within their definitive hosts. Microparasites are characterized by small size, short generation times, and a tendency to induce immunity to re-infection in those hosts that survive. The duration of infection is usually short in relation to the lifespan of the host, but there are important exceptions, such as the slow viruses. The key epidemiological variable, by contrast with macroparasites, is whether or not the individual host is infected.