Sudden Death-need help preventing it

[MikeWu]

One of my roosters looked very sick one day. He was all by himself in a roomy cage with lots of fresh air. He stopped eating and drinking. He died next day in the cage. I examined the body and found the belly skin turned bluish. I cut the belly open and saw the intestine all rotten. It seems some sort of Septicemia that caused its quick death. What could be the cause of the disease? He was alone in his cage for a long time so there's no outside contamination from other poultry. The only source of contraction might be form some wild squirrels (or birds) coming into the cage to feed on chicken food. There's no external physical injury.

Please help identify the disease so I can avoid future losses.

Thanks very much!