It is characteristic of many viral diseases of invertebrates that they may manifest themselves in both covert and overt forms, with drastically different effects on infected hosts. This difference between invertebrate and vertebrate systems has not been well studied, and yet it may be crucial in determining the outcome for an infected population, in particular in the presence of a vector that may catalyse a transformation from the covert to the overt form. We shall analyse such a system, using as an example deformed-wing virus infecting honey bees, in the presence or absence of varroa mites. Differences between Apis mellifera and Apis cerana will be considered.