Some species are responsible for serious human diseases, such as malaria, vectored by mosquitoes.
However, there are about 1200 species, out of 15,000 described, specific to and causing diseases in insects. One group, the Microsporidia, contains many species that have promise for biological control. Microsporidian infections in insects are thought to be common and responsible for naturally occurring low to moderate insect mortality. But these are relatively slow acting organisms, taking days or weeks to debilitate their host. Frequently they reduce host reproduction or feeding rather than killing the pest outright.