If the leering clown face of this microscopic image creeps you out, your instincts are correct: it's a deadly parasitic amoeba called Naegleria fowleri, and it's capable of devouring your brain from within.
While cases of parasitic meningitis caused by this tiny creature are fairly rare (four to five cases reported annually since 1962), one such occurrence has recently been reported by the Arkansas Department of Health. Apparently a 12-year-old girl was swimming in the Willow Springs Water Park in Little Rock when the parasite entered her brain.
National Geographic investigated the case further, consulting the Centers for Disease Control and Prevention. Epidemiologist Jonathan Yoder told them that the Naegleria normally feeds only on freshwater-dwelling bacteria, but in some cases it can develop appendages called “flagella” which enable it to swim more easily up a person's nose and into their brain. The resulting infection is usually fatal within a matter of days.
Now before you run screaming from the swimming pool, Yoder points out that this amoeba normally cannot survive in filtered or chlorine-treated water. If you do choose to swim in freshwater lakes, ponds or streams, the CDC recommends keep your head above the surface or use nose clips to keep water from entering your nose, as well as avoiding stirring up sediment at the bottom of shallow freshwater areas. They have more safety information at their official site.