Nosema bombi is a microsporidian, a small, unicellular parasite recently reclassified as a fungus that mainly affects bumble bees.
The parasite is believed to rely mainly on horizontal transmission between colonies via infected workers contaminating shared food sources such as pollen or nectar, but there is some evidence that it may also be transmitted vertically.
For example, Manlik et al. (2017) reported that N. bombi infection prevalence in buff-tailed bumble bees (Bombus terrestris) fluctuated between 2% (2010) and 81% (2003) in Neunforn, Switzerland.
[7] Under laboratory conditions, the fungus has been found to affect the survival and efficiency of adult individuals as well as the sperm counts of male offspring.
[8] There is some concern that higher infection rates among commercial colonies may lead to N. bombi spillover into wild bumble bee populations.