Further to JustWalking.
Hmmm... slugs and snails as food. Quite interesting really. On the face of it, they should be OK. They're just molluscs, after all. However...
Warning number 1. The University of California Santa Cruz has the banana slug as their mascot. Among other things, licking banana slugs can be a bit hallucinogenic apparently. Trust Santa Cruz students to work that out. Maybe someone out there knows why, but I'm suspecting it's diet. The little blighters are detritivores, including fungi. Who knows what chemicals get into the flesh.
Warning number 2. Helminth worms love freshwater snails. Schistosomiasis - considered by sources in Wikipedia as having economic effects second to malaria - loves snails. Galba snails are notorious for this
Warning number 3. Nematode worms love snails such as Achatina, which actually looks quite yummy. Hello... some forms of meningitis and other nasties mediated by Angiostrongylus.
I guess the first one can be dodged using a "don't eat anything colorful rule". The second ones by "If you're going to eat it - cook it really well", as JustWalking points out.

Those are just the ones I know of offhand. Any others? Shame this question had to arise. I kind of like escargot. In filo. With a white wine, butter, garlic, bechamel sauce.