Nosema apis was a nasty problem for queen bees, often causing early supersedure. From my reading of numerous studies, I get the hunch that the parasite is most effectively transmitted from the attendants to the queen during the chilling and stress of shipment or holding at room temperature (Lehnert 1973, Webster 2004). At that point, the queen can then become a “Typhoid Mary”—infecting the workers via the spores in her feces, which are licked up by the attendant bees (Loskotova 1980)