Florida health officials are urging swimmers to steer clear of stagnant water, which could be home to millions of microscopic killers.
Naegleria fowleri, an amoeba invisible to the naked eye, loves to lurk in warm, standing water, according to a warning released by the Florida Department of Health. And while it's usually harmless, it can cause a fatal brain infection if inhaled through the nose.
"Wear nose clips, hold your nose shut or keep your head out of the water when swimming, jumping or diving in any freshwater," the department said in a health alert. "Closing your nostrils may reduce your chance of becoming infected."
While exceedingly rare, Naegleria fowleri infections are almost always fatal. Only one person out of 128 infected in the United States between 1962 and 2012 has survived, according to the U.S. Centers for Disease Control and Prevention.
"After the start of symptoms, the disease progresses rapidly and usually causes death within about five days," the agency's website reads.
Early symptoms include a severe frontal headache, fever, nausea and vomiting, according to the CDC. But those can swiftly give way to a stiff neck, seizures, confusion and hallucinations - and the amoeba makes its way up through the nasal cavity to the brain.
Naegleria was eyed in the death of a Minnesota child last summer, when the state experienced a heat wave. And in the summer of 2011, the amoeba killed four people in Virginia, Florida, Kansas and Louisiana, all of whom had been swimming in freshwater lakes.
The CDC offers the following tips for summer swimmers: