The median age of the victims is 12, possibly because children and teenagers are more likely to play and swim in water.
The amoebas enter the human body through the nose after an individual swims or dives into warm fresh water, like ponds, lakes, rivers and even hot springs.
Lurking in fresh waters during the summertime, they're more likely to infect humans in July, August and September.
It's unclear why out of the millions of people who swim in the same fresh waters, a small fraction of people are infected by the amoeba.
"It's difficult to know," Yoder said. "It's not a disease that's easy to study because the end result is so severe. It's difficult to study in the lab."
Scientists speculate that the lack of certain antibodies could be why some children get infected, while others who've swum in the same water don't, said Francine Cabral, professor of microbiology at Virginia Commonwealth University School of Medicine.
The amoeba is not a parasite. A human is an "accidental end point for the amoeba after it's forced up the nose," Yoder said. It does not seek human hosts.
But when an amoeba gets lodged into a person's nose, it starts looking for food. It ends up in the brain and starts eating neurons.
"It causes a great deal of trauma and a great deal of damage," Yoder said. "It's a tragic infection. It's right at the frontal lobe. It affects behavior and the core of who they are -- their emotions, their ability to reason -- it's very difficult."
Early symptoms include headache, fever, nausea, vomiting and neck stiffness. Later symptoms include confusion, lack of attention to people and surroundings, loss of balance, seizures and hallucinations.
The amoeba multiplies, and the body mounts a defense against the infection. This, combined with the rapidly increasing amoebas, cause the brain to swell, creating immense pressure. At some point, the brain stops working.
Death typically occurs three to seven days after the symptoms start.
At hospitals, the infection is often mistaken for bacterial meningitis. Even when the diagnosis is made, the infection is difficult to treat.
The primary treatment for Naegleria infection is amphotericin B, an antifungal medication injected into the veins and brain.
But so far, only one person -- back in 1978 -- is known to have survived an infection, Yoder said.
Here are the CDC's tips for prevention:
* Refrain from activities in warm, untreated or poorly treated water, especially when water levels are low and temperatures are high.
* Hold the nose shut or use nose clips when swimming in warm fresh water.
* Avoid digging or stirring up underwater sediments while submerged in shallow, warm freshwater areas.