Viruses are obligate parasites which are completely dependent on a host cell for their replication. Lacking their own reproductive machinery, they usurp that of the cell which they infect. Viruses infect the larvae as well as the adult bees, and may dramatically reduce their lifespan. They consist of genetic material (RNA or DNA) protected by a protein capsule ('capsid'). Their genetic code is highly variable, causing rapid changes which make them hard to combat. More information on three of the most problematic viruses can be found in our Guide to Bee Health.