Infection rates for Legionnaires’ disease have increased nearly four and a half times since 2000, but experts aren’t sure why the illness is on the rise.

An outbreak of Legionnaires’ disease possibly linked to Disneyland has put a spotlight on the rise of a potentially deadly disease.

Yesterday, officials confirmed that three more people were diagnosed with the bacterial disease, bringing the total number of those infected to 15.

Eleven of those infected had been to the Disneyland park, and the other four lived or traveled to the Southern California city of Anaheim, where the park is located.

Those sickened by the disease were between the ages of 52 to 94, and 13 have been hospitalized after contracting the illness.

Two people, who had underlying health issues, died after contracting the disease.

Two cooling towers in the Disneyland air condition system, where Legionella bacteria were found and treated, have been shut down pending further confirmation that they are free of contamination.

The bacterial infection was discovered after it infected dozens of people who had visited Philadelphia for an American Legion convention in 1976.

The disease is spread via bacteria that grow in water and infect when inhaled via aerosolized droplets.

It’s been known to spread via fountains, air conditioning units, and grocery store produce misters.

This most recent outbreak is a fraction of the 6,000 expected cases of Legionnaires’ disease found in the United States every year.

This number has risen dramatically in recent years, increasing nearly four and half times since 2000 according to the Centers for Disease Control and Prevention (CDC).

Officials remain stumped by what exactly is causing this increase in Legionnaires’ disease diagnoses.

One possible reason for the increased number of diagnoses may have to do with medical technology.

In the early 2000s, doctors increasingly tested for Legionnaires’ disease via a new urine test.

Prior to that, doctors had to take samples from a patients’ sputum and grow bacteria in a laboratory to test for the disease, making it more difficult and time consuming to get a positive diagnosis.

A patient who showed signs of pneumonia could be given antibiotics, and as long as they recovered, there was no reason to search for a cause for their pneumonia.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, said these urine tests and more research on the disease have led doctors to increasingly look for Legionnaires’ disease in their pneumonia patients.

“There isn’t any doubt that we’re getting better at identifying it in patients, but also there seems to be a very real increase and folks are not sure why that is,” he told Healthline.

A changing climate can affect how easily bacteria can grow in water.

Legionnaires’ disease is caused by Legionella bacteria, an organism that grows in the natural environment and can multiply in warm water.

Disease diagnoses are generally more common in summer and early fall, when water is warm and conducive to growing bacteria.

“There’s been discussion about how much climate change or at least weather influences Legionella,” said Schaffner.

Last year was declared the hottest on record according to NASA, and the two years prior also were record-breaking for increased temperatures.

Schaffner, who is also part of a CDC research group studying the disease, pointed out that warmer weather could mean large air conditioning units are being used more often.

That could increase the likelihood of Legionella growing in these systems and spreading.

However, Schaffner said it’s still unclear if weather patterns are a reason that the disease is increasing.

Dr. Amy Edwards, an infectious disease expert at University Hospitals Cleveland Medical Center, said floods like the one that took place in Houston during Hurricane Harvey could also play a part in the increasing number of cases.

“With floods we expect a spike in Legionella, and so with the climate changing and flooding becoming more common, is that playing a role in it,” she told Healthline. “There are a lot of different things going on.”

The water systems in U.S. cities are increasingly falling into disrepair, according to the American Society of Civil Engineers, and an aging water system can be a haven for bacteria like Legionella.

In their 2017 “report card” on U.S. infrastructure, the American Society of Civil Engineers stated that the water systems of America earned a “D.”

“Drinking water is delivered via one million miles of pipes across the country,” they said in the report. “Many of those pipes were laid in the early to mid‐20th century with a lifespan of 75 to 100 years.”

In Flint, Michigan, a spike in Legionnaires’ disease was connected to corroded pipes that were damaged after improperly treated water was run through the system.

Schaffner explained that poorly treated water, aging pipes, or poor water pressure can create spots in the system where a “biofilm” can develop.

“The inside of the pipes can get very gunky and corroded… and in that organic material that’s grown up on the inside of [those] pipes, that’s where the Legionella can live,” he explained.

Poor water infrastructure means Legionella can grow and potentially infect a person via inhaled steam and droplets when they take a shower.

The population of the United States is not only aging, but also becoming more at risk for contracting a disease like Legionnaires’ disease.

Older people with lung problems or compromised immune systems are more at risk for developing the disease.

Young people who are healthy are unlikely to develop symptoms even if they are exposed to the same bacteria.

In addition, Edwards explained that paradoxically certain medical successes have made some people more vulnerable to contracting this bacterial disease.

People who’ve survived an organ transplant or who live with HIV or metastatic cancer have compromised immune systems. Decades ago, they may have been too sick to survive or to be out in the world, but now many of them can live normal lives where they might come into contact with Legionella bacteria.

“Classically, we always associated Legionnaires’ disease with smokers and people with chronic obstructive pulmonary disorder,” said Edwards. “It’s starting to be less true because more people with other kinds of conditions are living longer.”

Both Edwards and Schaffner said that despite research and speculation, experts remain unable to explain the dramatic rise of Legionnaires’ disease in the United States.

“We discussed this in this CDC research group several times, and the discussions just go round and round,” Schaffner said. “All the things we have mentioned might indeed have been contributors, but we don’t really know.”