I've seen the green muscle disease, and you can't miss it. It's flippin' gross. For the most part, you don't have to worry. In general, the disease process is pretty quick in birds. They usually die pretty quick if they're going to. In general, if I have one looking/acting sickly, I cull it and burn it just to be sure. It's not really common though, despite what people would have you believe about Cornish X's. Also, even if a sick one DOES slip through, and you butcher it, you'll probably be fine. Cooking will kill pretty much anything unsavory that might be in the bird... and does anyone think for a second that the huge poultry farms are keeping a close eye on all their birds and weeding out any that might be sick? I'd be shocked if half the birds at the grocery store didn't die of "natural causes!"