There is a disease called schistosomiasis, which is caused by a whole family of flatworms (could be hundreds but also thousends of different species of flatworms), and is a real problem in the tropics. And often lethal.
https://en.wikipedia.org/wiki/Schistosomiasis
If you read the wiki article you'll see that again the snails form an integral part in the lifecycle of all those parasites.
This is a CDC diagram of the worms' lifecycle:
This diagram talks about eating the fish, but you can also get flatworm infections directly through your skin, so swimming in infested water etc.