[GuidoK]

There are loads of parasites (mostly flatworms and roundworms) that use snails/slugs as their host in part of their life cycle.
Usually they creep into the snails tentacles where they move back and forth, simultaniously control its brain to creep to a high place. The movement in the snails tentacles mimic the look of a larva or caterpillar; favorite food of birds.
The snail/slug gets eaten, the roundworm/flatworm produces offspring in the birds stomach, gets shit out on leaves where snails eat the droppings or travel over it. Circle complete.
There are thousends of parasites that use such a cycle with the snail/slug. They're always the ones getting screwed.

Fascinating 1st year material at university studying biology.

Never eat a raw slug/snail. Also never eat a raw fish (uncooked or commercially frozen) thats not rated for consumption (like one you catch yourself). Fish are also full of parasites.