A virus is a small infectious agent that can replicate only inside the cells of living organisms. Viruses are pieces of genetic material that parasitize a host cell of a bee, forcing the cell to produce more viruses. Bee viruses are widespread and there are now more than 18 different viruses that can affect the bee. These include Deformed Wing Virus (DWV), Chronic Bee Paralysis Virus (CBPV), Kashmir Bee Virus (KBV), Black Queen Cell Virus (BQCV), Israeli Acute Paralysis Virus amongst others.
In general, viruses have not been too much of a problem for the beekeeper but recently there is strong evidence to suggest that the combination of Varroa and Nosema, especially Nosema ceranae, makes the bees more susceptible to viruses. There are a number of recent scientific papers that show that there may be a synergy between viruses and Nosema ceranae.