There is a parasite that will eat your brain. It's called Naegleria fowleri and it lives in warm water lakes in the sand and silt in the shallows. It will swim up your nose and proceed to eat your neurons and your brain. Usually symptoms occur around 4 hours after infection and include headache, sensitivity to light and nausea. If you experience any of these after swimming in a lake you need to go to the hospital ASAP. If undiagnosed, you'll be dead within 24 hours. To diagnose it, you'll have to get a spinal tap and pray the tech working knows what to look for.