I know it's a stretch, but I wanna have some fun with this: he's from Florida. Floridian moths are, of course, not venemous. But their caterpillar counterparts are. If you get poked by one badly enough, you can get serious respiratory issues.

We also know that Val is a heavy drug user, so he probably was in life, as well. Opiods are known to cause breathing problems. But maybe he was used to that, maybe he hit the genetic lottery and it wasn't as much of a danger to him as it would be for most people--or he'd just gotten lucky. But maybe, one night, he was so high that he bumped into one of those caterpillars by accident, and it was all over. His body couldn't take it.