I once carved a chicken and the breast on one side had a weird dark patch that smelled a bit off.
I actually wondered if it was some kind of cancer :S and did not eat said breast
It was AIDS. That's what the bad AIDS looks like. Bernard Matthews and Birdseye put the bad AIDS into their meat birds so that the meat is more tender and succulent. It's perfectly safe for human consumption though - as long as you follow the "ensure cooked throughout" instruction on the packet (that's why they are so clear about the cooking instructions!)