Snakes with parasites in the wild aren't necessarily sick. They (parasite and snake) usually live in harmony where the snake is able to keep parasite levels stable with it's own immune system. In other words, they are mere hosts. Now when they get stressed, immune system get majorly upset and throws everything out of whack and parasite levels increase to a point where it is harmful to the snake.
Although there are parasites out there which can and do kill snakes in the wild. But the majority of the ones we find while herping fall into the above category.
Kinda like Psoriasis which can be aggravated or surface all of a sudden when someone who has it gets really stressed.
It's all about the immune system.
The Herp Room
"The day I tried to live, I wallowed in the blood and mud with all the other pigs" - C. Cornell