Nosema disease in U.S. honey bees is caused by one of two (or both) fungi named Nosema apis and Nosema ceranae. Nosema species are obligate, fungus-like, intra-cellular parasites that are limited to specific hosts species. Nosema apis and N. ceranae cannot be reared in laboratory culture, as is possible with most bacteria and other fungi. They can multiply in living honey bee midgut, and perhaps other, cells. There is evidence that, like Nosema bombi in bumble bees, the N. ceranae may infect other honey bee tissues, but that remains to be substantiated.