Prior to varroa mites, most or all colonies may have had adequate  
levels of heritable resistance to a number of fairly common and  
ubiquitous viruses.  Only with some inbreeding did susceptibility to  
the common viruses surface in a few lines.   So at that level there  
must be heritable mechanisms that come into play to keep infections  
low, or at least prevent epidemics.  Individual immune responses?   
Social immunity via tossing out infected individuals?

After varroa, the scenario changes dramatically, at least for DWV.  It  
may be difficult to resist via individual immunity the highly virulent  
replicating forms of the virus activated by varroa feeding on pupae.   
Still a lot to learn it seems.  Saying that it is viruses that kill  
colonies is technically correct, but the killing is initiated by  
varroa.  It is analogous to saying that driver distraction is causing  
higher number of accidents, when the distraction comes from using  
electronic devices.

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