It's really about the vector. Varroa mites are parasites but just like ticks they carry pathogens. Varroa mites feed on the fat and hemolymph but they also inject viral particles into the developing bee. Deformed Wing Virus, DWV, is lethal to a mite infested colony. So when we talk about treating our colonies for mites, we are really managing viral loads. Healthy bees without mites can withstand DWV, just like we can fight off a cold/flu. But when the bees' immune systems are compromised they can succumb to viral infections. Just like people with immune-mediated disorders can succumb to pathogens that healthy people fight off. There are a number of parasite-pathogen partners in nature. One you are most likely familiar with are fleas that carry tapeworms. That's why your dog or cat needs to be wormed if they have fleas. The tapeworms eggs survive in the flea and when ingested the tapeworm eggs are released into your pets intestine. There the tapeworms develop into adult worms and you may see segments released into their feces. The varroa mite does not die from the virus it carries, it serves as the vector, it passes it along. The real lesson here is managing for a healthy bee colony so that the bees can "fight off" naturally occurring diseases like DWV.