Naegleria Fowleri

Naegleria Fowleri, also known as "Brain-Eating Amoeba," lives in the sediment of warm, freshwater ponds and lakes in the southern states of America. The amoeba has been found to be extremely dangerous and efficient, with a 97% death rate in its victims. When people go swimming, the amoeba gets into their noses and find their way to the brain. On average, it takes about 5 days for the amoeba to kill its victims, however, scientists haven't been extremely worried about it because chances of infection are extremely low. From 2005-2014 there have only been 35 cases in the U.S. 

http://www.cnn.com/videos/health/2015/08/31/brain-eating-amoeba-cohen-explainer-orig-mg.cnn/video/playlists/brain-eating-amoebas/