The thing is, you don't know that no harm was done. Once bacteria form a small colony inside a snake's body, it can live without causing any noticeable harm for months or even years. this is especially common in collections where the herps have been exposed to trace amounts of pathogens from other herps in the collection and the immune system now recognizes the bacteria as normal when really it can be deadly to that particular species. As long as the bacteria colony stays small enough, the snake's immune system just allows it to flourish.
Suddenly when the snake is stressed, too cold, is laying eggs, or has some other event which weakens its immune sytem the bacteria colony runs rampant and the snake sickens and dies within days. The only way you know what killed the snake is by doing a necropsy and culturing body tissues and fluids, then finding a pathogen which does not naturally occur in that species.
The Zombie Mama is here!