It doesn't happen often. But most summers, several Americans -- usually healthy, young people -- suffer sudden, tragic deaths from a brain-eating amoeba. What is this scary bug? How does it get to the brain? Where is it and how can I avoid it? WebMD answers these and other questions.

What Is a Brain-Eating Amoeba? Amoebas are single-celled organisms. The so-called brain-eating amoeba is a species discovered in 1965. It's formal name is Naegleria fowleri. Although first identified in Australia, this amoeba is believed to have evolved in the U.S. There are several species of Naegleria but only the fowleri species causes human disease. There are several fowleri subtypes. All are believed equally dangerous. N. fowleri is microscopic: 8 micrometers to 15 micrometers in size, depending on its life stage and environment. By comparison, a hair is 40 to 50 micrometers wide. Like other amoebas, Naegleria reproduces by cell division. When conditions aren't right, the amoebas become inactive cysts. When conditions are favorable, the cysts turn into trophozoites -- the feeding form of the amoeba.