Few things rank higher on the scale of "yuck" than having your brain devoured by hungry amoebas. However, if you are unlucky enough to acquire an infection from the microscopic animal Naegleria fowleri, then you can enjoy the distinction of having one of the world's most unusual types of encephalitis. The novelty won't last long, since most infections progress rapidly to death.
Amazingly, these carnivorous protozoans can be found in many warm swampy ponds, city water supplies or even hot water heaters. The cysts pass harmlessly through your digestive tract, but if the contaminated water gets flushed into your nose, then the amoebas can crawl through the porous cribriform plate at the roof of your sinus, following the sensory nerves that directly enter the brain. Within a few days, you may feel a dull headache, fever, stiff neck and sluggishness as the parasites begin dining on your central nervous system. There is no known treatment.
Naeglaria can infect victims who go swimming in contaminated water holes, or oddballs who enjoy using "neti pots"to irrigate their sinuses with tap water. But relax -- nobody has ever gotten infected from bathing, or swimming in chlorinated pools or salt water. There's no test for Naegleria, so it's pretty much trial and error to find a water hole that won't kill you.
If you have a headache the morning after skinny dipping it's probably a simple sinus infection -- or a hangover. Only a couple of cases of Naegleria are reported each year, so you have a thousand-fold greater chance of drowning, not to mention the risk from falling branches, water moccasins or getting a broken neck from messing around with that rope swing.
But if you decide to sooth your morning-after sinuses with a "neti pot," remember to avoid using warm tap water -- especially if you happen to live in Louisiana.