A 14-year-old boy died of a rare infection caused by a one-celled or-ganism believed to be common across North America.

Richard Davis died Friday at Florida Hospital of a form of encephalitis. Although he contracted the infection while swimming with friends in a drainage canal in Orange County on July 26, no one else was infected."We don't know why some people are affected and some aren't," said county epidemiologist Bill Toth.

Officials think the amoeba, called Naegleria fowleri, is found in all American freshwater lakes and streams, The Orlando Sentinel reported Saturday.

It usually lies harmlessly concealed in sediment, but if stirred up into the water it can enter a person's nasal passages and spread to the brain.