The ongoing outbreak of Legionnaires’ disease in Hopkins, which rose to 17 cases Thursday, is part of a dramatic national increase that so far has baffled doctors and epidemiologists.

The three new cases reported Thursday involved patients over 50 — an age group most likely to be susceptible to the pneumonia-like infection, the Minnesota Department of Health said. All have been hospitalized. One elderly person died as a result of the infection since the outbreak was first reported Sept. 9.

Health officials said more cases could emerge in the coming week because the infection has an incubation period of 10 days, and it could take a few more days for patients to see a doctor and have their cases reported.

Officials have disinfected two suspect industrial air conditioning systems in Hopkins and turned off a decorative fountain, each a potential source of the bacteria.

“We didn’t leave any stone unturned,” said Richard Danila, who is leading the investigation for the Health Department.

Hopkins officials say residents have taken the outbreak in stride. They’ve reported possible sources and cases to state health officials, and local companies quickly gave access to health investigators and disinfected their ventilation systems.

Nonetheless, there is a sense of unease among some.

“I think people are scared,” said Crystal Jacobson, manager of the Mainstreet Bar and Grill. She said attendance at the bar’s music events seems to be down, and some in the community fear that people are avoiding Hopkins.

“It seems to have taken its toll with the businesses around here,” she said.

Since 2000, the incidence of the pneumonia-like infection has increased fourfold, federal health officials reported earlier this year. Legionnaires’ now strikes up to 18,000 people annually, killing about one in 10. The most recent big outbreak struck New York City last summer, when 130 people were infected and 12 died, including an elderly Minnesota man who was visiting to watch a Yankees game, Danila said. It was his case that helped investigators find the source — the air conditioning system at the Bronx hotel where he stayed.

Minnesota also has seen a steady uptick in the number of cases, which hovered around 20 per year in the early 2000s and this year has surpassed 60.

But outbreaks like the one in Hopkins are unusual, Danila said.

Typically cases occur one or two at a time, he said, and the source of the bacteria is never found. The last big outbreak in Minnesota occurred in 1995 in Mankato and Luverne. Two people died and dozens became ill.

The disease was first named in 1976, when it struck a Legionnaires’ convention in Philadelphia.

After a lengthy investigation, researchers finally determined that the bacteria were growing in water in the hotel cooling tower and were spread through tiny droplets that were dispersed into the air and inhaled by passersby.

That’s why, when the first cases were reported among people who lived and worked in Hopkins earlier this month, state disease investigators immediately began looking for cooling towers, fountains and other systems that spray water.

“We were crawling on catwalks to collect water samples,” Danila said.

Experts stumped nationwide

Test results, however, are not yet complete, as the bacteria are “finicky” and slow to grow in laboratories, he said.

Danila said the source of the bacteria may never be identified. For one, the bacteria can live in pipes, fountains and cooling systems without making people sick — so investigators may find it in more than one place. On the other hand, industrial cooling systems contain so much water that investigators might not catch the bacteria in a small sample, he said.

Whether they identify the source or not, he said, investigators think it’s likely that the steps they’ve taken have already fixed the problem.

The Hopkins investigation provides a glimpse of why the growing number of infections nationally have stumped epidemiologists.

“It’s not completely clear what’s happening,” said Mike Osterholm, a professor and head of the Center for Infectious Disease Research and Policy at the University of Minnesota.

While reporting has certainly improved, that’s not the only reason the number of cases has climbed, he said. The U.S. Centers for Disease Control and Prevention says that aging buildings and water systems could be part of the problem.

But Osterholm said that new, greener construction standards may also play a role by creating conditions that the bacteria prefer.

“Minimizing temperature changes, making buildings more airtight and the need for more air conditioning — this is an area ripe for lots of study,” he said.

Now, scientists believe that a type of amoeba that thrives in the green slime that builds up inside cooling systems may be a host for the Legionella bacteria. When the amoeba dies, the bacteria escape into the water, Osterholm said.

At the same time, the nation’s aging population may simply be more susceptible to an infection that’s always been present. It strikes the elderly and those with compromised immune systems and weak lungs, he said.

“As baby boomers age we will see a higher risk of outbreaks,” Osterholm said.

For now, the best way to slow the increase in infections is rigorous maintenance and routine disinfection of water and air-conditioning systems, he said. The CDC and the construction engineering industry this year produced new guidelines for building managers on how to do that.

“Four decades after it was understood to be from a water tower, we still have huge gaps in our understanding,” he said.