They Eat Your Brain

Using a neti pot.  From Oprah.com

Among some people, it is considered healthy to rinse out your sinuses with water, using a 'neti pot'.  When performed with a sterile solution, the worst that seems to happen is an increase in infections as the protective mucus layer is removed from the sinuses.

However, several people have died recently from using water contaminated with "brain-eating amoebas" — known to scientists as Naegleria fowleri.  These single-celled nasties live in fresh water, especially down south where the water is warm.  If you swallow N. fowleri while swimming they die in your stomach acid.  If they get up your nose, they travel into your brain, multiply, eat your brain, and kill you.  Since 1962, there’s only one documented case of a US patient surviving PAM.

So ...  In 2010 and again in 2013, people died from N. fowleri infections in Minnesota.  Minnesota, for those not looking at a map, is not down south.  The organism is moving north with climate change.

Short of avoiding fresh water swims altogether, public health officials recommend plugging one’s nose during dives or periods underwater. Tightly.

You can see more horrifying information at the Weather Channel, and The Verge.