MythBusters did a show on piranha. IIRC, they actually sat in a giant tub full of them and never got bit. They attack in response to stimulus. If they’re attacked or there’s blood in the water, they’ll go into a frenzy. (Biological) females on their monthly should avoid swimming in the ocean where sharks are known to be found for the same reason.
If it’s not one thing, it’s another,
Getting eaten by piranhas, or having a candiru fish swim up you’re urethra.