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It literally devours the brain, killing its host in less than two weeks. The vague initial symptoms like headache, fever and nausea quickly give way to neurological problems, and worse. Out of three people infected this summer in the United States, just one miraculously survived — Kali Hardig, a 12-year-old girl from Arkansas. She’s only the second survivor out of 128 infections during the last 50 years in the U.S. In Louisiana late last year, officials flushed two water systems with additional chlorine after tests confirmed its presence. What is it that has people feeling so spooked?

It’s Naegleria fowleri, commonly known as the “brain-eating amoeba.” A free-living, single-celled protist less than half the width of a human hair, it strikes humans very rarely, but with a fatality rate greater than 99 percent. N. fowleri is found in warm freshwater ponds, lakes, rivers and hot springs, especially in the southern United States during the summer.

Drinking contaminated water will not cause an infection, but if the amoeba manages to enter a body through the nasal passages — when someone is swimming, for example — it will move very quickly along the olfactory nerve and make its way to the brain, evading the host’s immune system and bypassing the blood-brain barrier, which is the brain’s only real protection from pathogens. Once it’s in the brain, N. fowleri starts eating — and is almost impossible to stop.

The amoeba “probably goes for the brain because it’s looking for tissue with a high oxygen content, and brain cells have a very high oxygen content,” said Jennifer Cope, a researcher with the U.S. Centers for Disease Control and Prevention.

After those initial, flu-like symptoms, patients experience what’s called primary amoebic meningoencephalitis. The patient’s neck starts to stiffen, and he or she loses the ability to taste and smell. Delirium and even hallucinations can occur. Victims lose the ability to control their muscles, followed by seizures. Shortly after falling into a coma, the patient dies.

Francine Marciano-Cabral, an immunologist at Virginia Commonwealth University, explained that the amoeba successfully repels the body’s various immune responses, such as antibodies and various compounds designed to destroy pathogens. The amoeba’s speedy rate of travel through the nervous system also helps it evade the immune system.

“It’s so deadly,” Marciano-Cabral said. “We think that if an animal in the environment becomes infected, it will die and release a highly pathogenic version of the amoeba, which attaches to cells more rapidly than a weakly pathogenic or non-pathogenic form, but we don’t have enough research to know for sure.”

Because N. fowleri is so fatal so quickly, it is very difficult for researchers to understand even the basics of how it attacks humans, let alone how doctors should try to counter it, Marciano-Cabral said. As of now, an anti-fungal medication called amphotericin B is the best hope for survival. Its exact mechanism against N. fowleri is not well understood, but it is thought to work by degrading the amoebic cell membrane.

Adding to the treatment difficulties is the fact that the infection does not produce symptoms for five days, and initial symptoms, like headache and fever, are unspecific. Making a positive diagnosis requires procuring a sample of cerebrospinal fluid via a spinal tap, a highly invasive procedure that physicians do not undertake lightly.

So how was Kali Hardig able to survive? She was treated with amphotericin B as well as an experimental drug called miltefosine, which is currently not approved by the U.S. Food and Drug Administration. “We definitely don’t think [miltefosine] is the silver-bullet cure for it, but it seems to certainly have made a difference in this case this summer,” said Cope, the CDC researcher.

One interesting characteristic that Hardig and other survivors of the infection share is that they tend to have a higher immune response, possibly due to prior exposure to the amoeba that had not led to infection, according to Cope. N. fowleri “is pretty common in the environment, so we actually think people come into contact with it much more frequently than they are infected,” she said.

It’s also possible that a less virulent strain infected Hardig, Marciano-Cabral said.

Although there has not been any research gauging the effectiveness of any preventive measures, the CDC recommends many kinds of common sense practices, including boiling water before practicing nasal irrigation and using nose plugs or keeping the head above water when swimming in natural waters. In rare instances, N. fowleri can be found in unchlorinated swimming pools, so always be sure the water has been treated before swimming.

The amoeba was recently found in two water treatment systems in Louisiana: one from St. Bernard Parish, where a 4-year-old boy died of infection in August, and one from DeSoto Parish, where a 51-year-old woman died from infection in 2011. Officials responded by increasing chlorine levels at water treatment facilities in both parishes for several weeks, as well as following up with detection tests for N. fowleri. While residents have little to worry about since the amoeba must travel up nasal passages to cause infection, both Marciano-Cabral and Cope noted that the severity of the disease and the region’s history with infection warranted an additional amount of chlorination.

Despite the scary nature of the brain-eating amoeba, the CDC points out that the 128 known infections since 1962 make N. fowleri an extremely rare risk, although for reasons that are not quite understood. For comparison’s sake, there were nearly 39,000 drowning deaths in the U.S. during a recent 10-year period from 1996 to 2005. So while anyone who swims outdoors or handles contaminated water should exercise caution, there is no need to be an alarmist. If you take the appropriate precautions, go ahead and enjoy an outdoor swim — and try not to think about the very remote possibility that a voracious microorganism could invade your nasal passages and head straight for your brain, in search of a deadly meal. Om nom nom nom…