Cases of Legionnaires' disease reached a record high in 2018 - a more than eightfold increase since the numbers began to climb nearly two decades ago.

The Centers for Disease Control and Prevention recently reported 9,933 cases in 2018 of Legionellosis, which includes both Legionnaires' disease and Pontiac fever. Legionnaires' disease made up the vast majority of cases, according to the CDC.

And the problem may be exponentially larger than what's reported to public health officials. The National Academies of Sciences, Engineering, and Medicine estimates that as many as 70,000 people may suffer from the disease each year, according to a report released in August.

Hot tubs, hotels and hospitals across the U.S. continue to be hotbeds for the potentially deadly disease, which people contract after inhaling mist or water droplets contaminated with Legionella bacteria. It causes severe pneumonia-like symptoms and kills 10% of those sickened. Nearly one-quarter die if they contract the disease in a health care setting.

Water that sits stagnant in old pipes, cooling towers or hotel rooms can become a breeding ground for the dangerous bacteria to grow and fester when water is not cool or hot enough. Experts point to a number of causes for the explosion of cases across the country: decaying infrastructure, an aging population, greater testing for the disease, water conservation efforts and even climate change.

National news has been transfixed by the sometimes deadly cases around the country, from the four fairgoers killed and nearly 150 sickened by a hot tub exhibit at a North Carolina state fair in September to multiple hotel guests struck ill at the Sheraton Atlanta Hotel this summer.

But fighting the problem is an expensive undertaking. The annual cost of treating Legionnaires' disease, based on hospitalization claims, was $434 million in 2012. It has likely grown with the uptick in cases.

Flushing out water systems or even redoing them to get rid of the bacteria costs far more - California alone has spent $8.5 million this year cleaning up an outbreak at a Stockton prison that killed one inmate. For facilities, the consequences of finding the bacteria can be catastrophic, even if not deadly. The water often must be shut off, leading to negative media coverage, the potential for lawsuits or financial ruin.

In Washington, D.C., the public psychiatric hospital St. Elizabeths relied on bottled water for weeks this fall after Legionella bacteria was detected, and some patients filed a suit claiming "inhumane, unsafe and medically dangerous conditions that risk the health, mental health and safety" of patients. The suit also alleged, "Patients cannot shower, wash their hands or use the toilets regularly. Fecal matter, urine and menstrual blood are accumulating in the bathrooms."

The remediation alone has cost more than $1 million, according to The Washington Post.

The fear of that kind of financial hit leads building managers to resist testing or actively looking for Legionella, said Laura Ehlers, who directed the study for the National Academies. And even when facilities have water management plans in place to prevent the bacteria, it still is still showing up.

As cases were reported in Missouri last year, state officials investigated 61 hospitals, nursing homes and hotels from last fall until late this summer, according to a document Kaiser Health News obtained through a public records request. Seven of those facilities tested positive for Legionella bacteria, even though each had a water management plan in place.

Since 2017, the Centers for Medicare & Medicaid Services has mandated that health care facilities have such water management plans. But even with a good water management plan, Legionella risk is ever present, according to Jonathan Garoutte, administrator of the Missouri health department's section for environmental public health.

"Legionella finds places within large complex water systems to hide," he said.

Following a 2015 Bronx, N.Y., outbreak, which killed 16 and sickened 138, the CDC redoubled its efforts, CDC epidemiologist Chris Edens said. The officials are finding that as temperatures rise and summer extends, that could spur more bacterial growth.

"If devices like air conditioning units are used for longer periods of time, that could certainly pose a risk," Edens said.

Ehlers also pointed to an unusual source: the green building movement. She said LEED-certified buildings and other efforts to conserve water often hold such water at temperatures that are unsafe.

"It's made a growth dish for Legionella," she said.

As cases have risen, the CDC has increased communication about outbreaks, created a water management toolkit and helped CMS set a 2017 rule requiring health facilities and nursing homes to have a water management plan to reduce the risk of Legionella. However, officials updated the rule in 2018 to clarify that CMS does not require health care facilities to test for Legionella or other waterborne pathogens.