The genus, circumscribed by Swiss botanist Carl Nägeli in 1857, contains 81 species.

[1] Most parasitise insects and other arthropods, and the best-known Nosema species parasitise honeybees, where they are considered a significant disease by beekeepers, often causing a colony to fail to thrive in the spring as they come out of their overwintering period.

[2] Nosema locustae, which parasitises locusts and grasshoppers, and Nosema grylli, which parasitises crickets, have been transferred to Paranosema, or in the former case Antonospora.

Nosema algerae, which parasitises anopheline mosquitoes, has been transferred to Brachiola.

Studies of DNA sequences imply that the boundaries between the genera Nosema and Vairimorpha are incorrectly drawn.