Our bodies fight off countless microscopic attackers all the time.
But one little intruder has earned a notorious reputation for bypassing our natural defenses, burrowing into our skull and destroying our brains -- almost always with fatal results.
Public-health officials and scientists across the South, including Florida, are on alert because of Naegleria fowleri, an amoeba that lives in freshwater and becomes active in the heat of the summer.
Infections are rare.
But with recent cases in Florida and Arkansas drawing attention to the killer amoeba, the Orlando Sentinel recently asked experts how N. fowleri attacks and what we can do to stay safe.
What's the best way to avoid infection?
The amoeba is found everywhere in freshwater, including lakes, rivers, and hot springs, so the best way to reduce risk is to avoid such bodies of water. That's especially true in Florida during the summer. The peak months for infections are July, August and September. Southern states have the most cases.
Florida led the nation with 33 cases between 1962 and 2012, according to the Atlanta-based federal Centers for Disease Control and Prevention. Texas came in second with 31 cases during the same period.
What should I do in freshwater?
If you can't avoid freshwater, make sure to keep your head out of the water. Hold your nose shut or use nose clips. Avoid digging in or stirring up sediment. That could release more organisms to the surface and eventually into your body.
How does it attack?
N. fowleri attaches to the cells that line the inside of our noses, also known as the "nasal mucosa," according to Dr. Nicole M. Iovine, director of the Antimicrobial Management Program in the Division of Infectious Diseases & Global Medicine at the University of Florida.
What happens next?
It produces toxic proteins that kill the mucosa cells, allowing it to invade deeper. It moves on its own, using two whip-like structures called flagella. Then it takes a ride along the olfactory nerve, which gives it a direct route into the brain.
Why does it attack our nervous system?
Iovine said certain areas of the body aren't as well protected as others. When N. fowleri gains access to the central nervous system, for example, it establishes infections before immune responses can kick in.
Why doesn't our immune system kill it?
There are some parts of the body that are considered "immunologically privileged," Iovine said. That means our immune system doesn't have instant, easy access to those areas compared to better-protected parts, including skin. Other examples of immunologically privileged sites include the eyes, testes and placenta.
What does it eat?
As a "free-living" organism in the environment, the usual diet consists of bacteria, yeasts and other small microorganisms found in the water it lives in. "When presented with other food sources, such as human brain tissue, it can also digest this food source and proliferate," Iovine said in an email.
Does the disease have a name?
Yes, it's called primary amebic meningoencephalitis, or PAM. The rare disease is almost always fatal. Only two people in North America out of 128 have survived infection from 1962 to 2012. That doesn't include an Arkansas girl this summer thought to be the third survivor.
What are the symptoms?
The symptoms are sometimes misdiagnosed because they are similar to bacterial meningitis. The first stage of PAM includes severe headache, fever, nausea and vomiting. Second-stage symptoms include stiff neck, seizure, hallucinations and coma. Symptoms start one to seven days after exposure and death follows one to 12 days after symptoms.
What is the treatment?
Several drugs are proven to fight the organism in laboratory conditions but the effectiveness in humans is unclear. That's because almost all infections have been fatal.
Has this been a problem in Central Florida?
Yes. A 16-year-old Brevard County girl, Courtney Nash, died from a suspected PAM infection after sheand her family swam in the St. Johns River in August2011.
A 22-year-old man died from it in Orange County in 2009. Three Central Florida boys were killed in 2007.
What do we know about the current cases?
Zachary Reyna, 12, was infected while knee-boarding in a ditch near his LaBelle home in South Florida on Aug. 3. The child is being treated in Miami Children's Hospital intensive-care unit.
A 12-year-old girl from Arkansas, Kali Hardig, was infected a month ago after swimming in Willow Springs Water Park in Little Rock and has recovered.
Doctors have permission from the CDC to give Zachary a drug that worked for Kali. She is considered the third well-documented case of a person in North America surviving the infection since 1962.
Information from the Miami Herald and CDC was used in this report.