Mutualistic Symbiosis Wow that's a mouthful. At any rate that what varroa mites and deformed wing virus are to each other. In a few words, these two bee diseases work together and both benefit. For more detail read the link below.
The first article referenced the following article. It states that Apistan at least temporarily increases the virulence of DWV while killing the mites. In short, the Apistan may kill the mites, but it sets up the hive for a severe DWV infection. This increase in virulence did not occur when other natural miticides (formic and oxalic aicd) were used. This paper is dated 2012, so it may be outdated.