It's a virus, it wipes out populations of crickets. If you have had crickets that had the virus and you get new crickets without sterilizing where you keep your crickets you will spread it to your new crickets. That is why many have switched over to the black crickets that do not seem to be effected by the virus. However, they have their own problems because they are aggressive.
Here's an interesting article about the cricket virus. It's a very devastating virus that affects only common brown crickets and can wipe out entire colonies in days. Took the company in the article out of business in 2010.