Grasserie virus is what they're referring to. In the worst case scenario, the worms turn gooey and black and disgusting and then die. Less severe cases can reach large size, but they're stunted and may or may not pupate. I've heard that sick worms can cause problems with beardies, but I don't know if the virus causes the problem or if bacteria attack the immune-compromised worm and then cause problems for the beardie. Actually, I don't know if either way has been proven or if it's just hear-say. As far as I know, all the major suppliers are clean these days, so if you want to order, you'll be okay.