Nosema is a disease that affects adult honeybees. It is caused by a unicellular parasite. The parasite lives on the tissue in the midgut. Nosema is prevalent world-wide. The disease also occurs frequently in the Netherlands and Belgium. Nosema apis can cause substantial damage. Since 2008 Nosema ceranae has also been present in the Netherlands. This species, probably an exotic, is present in virtually all bee colonies, but it is unknown how and whether it causes disease or mortality in bee colonies in the Netherlands.

Nosema apis has been known for years in beekeeping, but now Nosema ceranae is also a potential problem. Nosema apis and Nosema ceranae are Microsporidia. Microsporidia are obligate intracellular parasites. This means that they cannot live and reproduce outside the host cell. Microsporidia live in cells of insects and other invertebrates. Quite recently, Microsporidia parasites were also discovered in humans (Barbancon 2007).

Nosema apis usually strikes in the spring. Long-lasting cold and rainy periods can result in insufficient pollen being produced; as a result bees become susceptible to this intestinal parasite. This can lead to mortality and dysentery. The symptoms of Nosema infestation are also characterised by the presence of dead and cramped bees inside and in front of the hive.

The lifecycle of Microsporidia includes the spore stage, during which the parasite can survive for long periods outside a cell. The spread from cell to cell and from one organism to another takes place in the spore stage.