Nosemosis is the most common disease of adult honey bees. It is caused by unicellular, intracellular parasites, so-called microsporidia, which are closely related to Fungi. It is spread through spores that are viable for several years. Two species can be differentiated: Nosema apis and Nosema ceranae, which can only be distinguished by PCR, but differ in pathogenicity. The pathogens infect the midgut cells and thus lead to yellowish diarrhoea. Affected animals are often unable to fly and the abdomen is bloated. Many times, symptoms are rather non-specific, the bees are weak. Nosemosis is a multifactorial disorder, which means outbreaks of the disease only occur when there are other adverse conditions involved, such as cold, other illnesses, etc. Hence, nosemosis is potentially curable by resolving the other factors. Due to the resistance of the spores, it is often difficult to completely eliminate the pathogens. As with many bee diseases, transmission occurs through the bees themselves (drifting or robbing) or through the beekeeper.