After the death of a 9-year-old boy Tuesday, officials moved quickly to close Stillwater's Lily Lake, with signs warning visitors to "Stay Out of the Water" and yellow "Danger" tape.

Jack Ariola Erenberg is the second child since 2010 to die of a rare brain infection believed to have come from an amoeba in the 36-acre lake.

"This is an incredible, tragic event," said Michael Beach, an author and a leading researcher on the amoeba for the Centers for Disease Control and Prevention (CDC) in Atlanta.

Still, Beach said, he and other national authorities wouldn't urge that all swimming stop in natural bodies of water, given the benefits and the extremely low risk. Officials do not want to minimize the deaths, Beach said, but they also are not willing to tell hundreds of millions of people to stop visiting lakes, streams and rivers.

Beach and others are trying to develop a practical, fast way to check water for the organism, known as Naegleria fowleri. It's the second time that a Minnesotan has died of meningitis caused by the microscopic amoeba. Though such infections are rare worldwide, 7-year-old Annie Elizabeth Bahneman of Stillwater also died after swimming in the same lake in 2010.

Officials at the Minnesota Department of Health are awaiting confirmation from the CDC but are 99 percent certain that the boy's death was caused by the amoeba, said Richard Danila, assistant state epidemiologist.

Jack's father, Jim Ariola of Wyoming, Minn., said he wouldn't have let his son swim there if he had known another child died. He feels that warning signs should have been posted at the lake.

"You can't keep your kids out of lakes, you know," Ariola said. "Who all has a pool? When I was little, I swam in that lake because I had hockey camp right there at Lily Lake. If they open the beach, I'm sure kids would be there today."

On the radar since 1962

The amoeba became known to health experts as a disease organism in 1962, Beach said. It's been found in water heaters, poorly disinfected pools, neti pots that people use to cleanse sinus cavities, lakes, streams, ponds and elsewhere.

Of 125 known U.S. victims in the past 50 years, only one has survived.

From 2002 to 2011, 32 infections were reported nationwide, according to the CDC. Of those, 28 people were infected by recreational water. Two people were infected by performing nasal irrigation using contaminated tap water, and two others were infected by water from an untreated geothermal drinking water supply.

The infections are caused when water goes up the nose, enabling the one-celled organism to crawl into the brain. Much about the scum-eating amoeba is unknown, but it thrives in warm water and proliferates in prolonged heat waves, Beach said. Most of the cases have been in Southern states.

Jack's death is raising new questions for perplexed national researchers, who are trying to develop ways to check bodies of water for the organism, which appears to be moving northward as lake temperatures slowly rise. Nobody knows how widespread it is in Minnesota, Beach said, because there's no good way to check for it.

Officials can't tell parents that there is no risk, but chances are very, very low of contracting such an infection, Danila said. There are ways to reduce that risk, including by wearing nose plugs, keeping heads above water and not diving.

"We deal with other risks such as boating accidents and drowning and everything else on a regular basis when we take part in aquatic activities, and so we have to put this into that same risk-reduction kind of thinking that we do for all of the other activities we do, and particularly in Minnesota," Beach said.

"I mean, you're full of lakes, you're all about aquatics, whether that's swimming or canoeing, kayaking or boating. Think of all the water exposure that's occurring on a regular basis."

Lily Lake's beach will be closed for the rest of the warm-weather season. Over the winter, health officials will decide what needs to be done regarding public safety, Danila said.

Became ill while camping

Ariola said Jack had fallen ill and become dizzy while camping with his mother and other family members in the Grand Marais area, on the north shore of Lake Superior, and that she took him to a hospital there.

At first, the doctors though it was flu, and Jack returned to camp.

By Saturday, he was sicker and seeing double, with a bad headache, so his mother took him back to the hospital, the father said. A medical helicopter flew the boy from Grand Marais to Duluth "and from there it was all downhill," his dad said.

Jack loved swimming, and he had wrestled at his old school. He was to begin fourth grade this year at Lily Lake Elementary in Stillwater, where he lived with his mother. He was to attend his first hockey practice of the season Wednesday evening for a Forest Lake team where he played goalie, his dad said.

"Let your family know you love them," Ariola said. "You never know when something like this could happen."

Joy Powell • 651-925-5038