It wouldn't surprise me if they link it to a virus. And, they're only detected the one virus. I'm not sure how much virus work has been done with bees, but if you use polymerase chain reaction? ...PCR testing, the agents have to be developed after you suspect the virus is there, in order to detect the virus.
Shrimp farming has suffered huge mortalities on farms in areas where shrimp from other areas, like SE Asia, were introduced into the Americas. Kind of the same effect as when the first white people sailed to Polynesia and killed the locals with flus and colds. They were exposed for the first time and had no resistance.
With the introduction of the African bee and the fact that Africa seems to produce some bada$$ organisms (cattle egrets, tilapia mozambique, AIDs, Ebola, some really bad malarias) it's possible a virus that piggy backed in with the resistant Africans could be the cause of the disappearing bees. But, it's manageable once it's determined. Specific pathogen free and resistant lines can be bred.