Studies suggest that N. fowleri amoebas are attracted to the chemicals that nerve cells use to communicate with one another. Once in the nose, the amoebas travel through the olfactory nerve into the frontal lobe of the brain.

This appears to happen most often when people are diving, water skiing, or performing water sports in which water is forced into the nose. However, infections have occurred in people who dunked their heads in hot springs or who used untreated tap water to cleanse their nostrils.

If you were to drink a glass of water infested with naegleria, you would not get a brain infection. Infection occurs only after water (or perhaps dust) containing the amoeba gets into the nose.

"It is normally eating bacteria in its natural environment, but for some reason it does use the brain as a food source when it gets into humans," Yoder tells WebMD.

How Frequently Do People Get Infected by Brain-Eating Amoebas?

Even though N. fowleri amoebas are relatively common, they only rarely cause brain disease. N. fowleri disease, known as primary amoebic meningoencephalitis or PAM, occurs from zero to eight times a year, almost always from July to September.

Worldwide, there have been some 400 reported cases. There have been 35 reported cases in the U.S. since 2001. Yoder and colleagues were able to identify 111 PAM reports in the U.S. from 1962 to 2008.

However, some cases may be unreported. A study in Virginia that looked at more than16,000 autopsy records from patients who died of meningitis found five previously unreported cases of PAM.

"I am sure we are missing some cases," Yoder says. "But these are pretty tragic infections, often involving children, so doctors and pathologists are motivated to find the cause."

Studies suggest that many people may have antibodies to N. fowleri, suggesting that they became infected with the amoeba but that their immune systems fought it off. It's not at all clear how often this happens.

"We have asked ourselves, 'Is this a rare infection that is always fatal, or a more common one that is only sometimes fatal?' We don't know the answer," Yoder says.

But in a 2009 study, Yoder and colleagues suggested that the common finding of antibodies to the amoeba in humans and the frequent finding of N. fowleri in U.S. waters indicate "that exposure to the amoeba is much more common than the incidence of PAM suggests."