Much rarer and more deadly is Naegleria fowleri, aka the brain-eating amoeba. This single-celled protozoan usually minds its own business, but occasionally infects people when they inhale water through the nose. "It can enter the brain and reproduce, causing severe and usually fatal brain damage," John Hawdon, vice president of the American Society of Parasitologists and a researcher at the George Washington University in Washington, D.C., said in an email. Most of these infections have happened in southern states, when people swim or dive in warm freshwater like lakes, rivers, or hot springs. "The amoeba thrives in very warm or hot water, so it is more common in the south, but there have been cases in northern states as well in the summer," Hawdon says.