The researchers detected about 450 cases in Atlanta-area hospitals over five years. About a third of the patients died in the hospital.

The findings, published this week by the U.S. Centers for Disease Control and Prevention, come from an attempt to better gauge how common these kinds of illnesses are. The government does not require doctors to report the illnesses.

“It’s kind of the first time we’ve had an estimate like this,” said the CDC’s Dr. Jeremy Gold, the study’s senior author.

And it’s important to understand it now, because the problem may grow in the future, he added. Mold exposure is a recurring problem in the wake of hurricanes, floods and other natural disasters influenced by climate change, researchers have found.

“I agree that the frequency of fungal infections will likely increase with climate change,” said Dr. Jeffrey Jenks, an infectious disease researcher at Duke University.

Meanwhile, there’s a growing population of older people who are more susceptible to such illnesses, Gold said.

Invasive mold diseases are rare infections caused by certain species. Most people who get infected inhale mold spores, but they can also enter the body through cuts or wounds or from contaminated medical equipment.

The spores don’t cause health problems for most people, but they can be life-threatening to some, including the elderly, cancer patients and others with weakened immune systems.

Previously, CDC researchers estimated more than 15,000 hospitalizations a year could be tied to invasive mold disease.

To better understand how common cases might be at a local level, the research team worked with four Atlanta-area hospitals and their affiliated outpatient clinics to look for potential cases from 2020 to 2024.

They initially identified nearly 1,000 potential cases, but winnowed the number down to about 450 by ruling out patients who may have had signs of exposure but didn’t qualify as having invasive mold disease.

The researchers found that hospitals can expect to see 3 to 5 cases per year for every 100 inpatient beds. The rates were higher at larger academic hospitals that tend to see more complicated illnesses.

Most patients had impaired immune systems or — in a number of cases — were weakened after falling sick from COVID-19.

Mold exposures can occur in hospitals or other healthcare facilities, but the study did not try to determine where each patient was first exposed, Gold said.

It’s hard to generalize what was seen in the Atlanta area to other parts of the country where mold may be more or less of a problem, he said. But it should give hospital administrators at least a rough idea about whether the number of cases they see is what might be expected or whether it’s an outbreak, Gold added.

Jenks called the study’s findings “valuable,” but said “seeing results from similar studies done in different parts of the United States would be helpful.”

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.