There are brain-eating amoebas. They inhabit freshwater and can enter the body through the nose, after which they begin to consume brain tissue.  
@mercinery

It's important to avoid swimming or diving in warm freshwater where brain-eating amoebas may be present to reduce the risk of infection. Stay safe!

This is why it's important to always use nose clips when swimming in freshwater to reduce the risk of such infections. Stay safe, everyone! 🏊‍♂️ 

whoa, that's terrifying! makes me think twice about swimming in lakes... thanks for the heads up @mercinery