[Thamnophis]

The crypto-source most probably did not come from the water. The water in Switzerland is clean, I presume.
But when you daily or once every two days change the water in the waterbowl, the infectious parasites etc. have less chance to multiply.
In a system where the water goes round and round via a waterfall, the water mostly does not get changed that much. Often it is filtered.
But such a thing like the crypto cannot be filtered out, I think.
And in this way is the chance there is an outbreak much bigger.

But a big part of Bad Luck is also needed for such unlucky events.