Naegleria fowleri
, a free living amoeba which can expose to the human brain through the nose, is almost always fatal. It is commonly found in the bottom of freshwater in lakes and ponds. It can cause Primary Amoebic Meningoencephalitis (PAM or PAME) in humans characterized by rapid onset of coma and death. These killer protozoa have claimed the lives of three boys this summer in the lakes around Florida's Orlando.
The amoeba can be detected by an MRI and it can be treated with antibiotics if caught early enough, but there is no consistently effective treatment currently. So, swimming in or jumping into fresh water is best avoided. Chlorinated swimming pools are safe from the perspective of contracting PAM.
Source: Deadly amoebas lurk in Florida lakes @ CNN Killer in the Lake -- Amoeba Called Naegleria @ Clinical Cases and Images