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Originally Posted by GuidoK

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.

I'm familiar with this one... some people catch it by touching dog or cat shit