When you finally return to work after the lockdown, coronavirus might not be the only illness you need to worry about contracting at the office.

Office buildings once filled with employees emptied out in many cities and states as stay-home orders were issued. These structures, normally in constant use, have been closed off and shut down, and health risks might be accumulating in unseen ways.

“The buildings aren’t designed to be left alone for months,” said Andrew Whelton, an associate professor of civil, environmental and ecological engineering at Purdue University.

Whelton, other researchers and public health authorities have issued warnings about the plumbing in these buildings, where water may have gone stagnant in the pipes or even in individual taps and toilets.

As lockdowns are lifted, bacteria that build up internally may cause health problems for returning workers if the problem is not properly addressed by facilities managers. Employees and guests at hotels, gyms and other kinds of buildings may also be at risk.

The biggest worry is Legionella pneumophila. The bacteria can cause Legionnaires’ disease, a respiratory condition. It leads to death in about 1 in 10 cases, according to the CDC. The National Academies of Science, Engineering, and Medicine estimates that over 52,000 Americans suffer from the disease each year.

A single small outbreak can sicken many people. During the water crisis that started in Flint, Mich., in 2014 after the city changed its water source and officials failed to inform the public of water quality problems, many people became sick. The crisis was linked to the deaths of 12 people from Legionnaires’ disease.

After an outbreak at the North Carolina Mountain State Fair in September, 135 people contracted the disease and four died, according to the state’s department of health and human services. Investigators blamed a hot tub exhibit that sent Legionella through the air and was inhaled by passersby.

Most worrying, Legionnaires’ disease tends to affect people with compromised immune systems.

“COVID patients and survivors could be more vulnerable to this, so when they go back to work we might be concerned about another infection,” said Caitlin Proctor, a postdoctoral fellow at Purdue who, along with Whelton, conducted a study that has been accepted for publication in the journal AWWA Water Science examining risks from water stagnation during the coronavirus lockdown.

Once forming in a building’s plumbing, Legionella can be dispersed through the air when toilets are flushed. Even turning on taps, as employees wash their hands to limit the spread of the coronavirus, can send water droplets into the air that carry Legionella.

Typically, facilities managers reduce the risk of bacteria by pouring small amounts of disinfectant into a building’s water systems. But when the water is left stagnant for too long, the disinfectant disappears.

“Even just after a weekend, disinfectant can be gone in some buildings and the water is vulnerable to contamination,” Whelton said.

Shutdowns in the U.S. began in mid-March, meaning some buildings have now been closed for two months.