There’s a long history of natural disasters making people sick. Reports range from cases of Valley fever after the Northridge earthquake in California in 1994 tossed dirt containing spores of Coccidia bacteria into the air, to aspergillus infections caused by victims of the 2011 Japanese tsunami aspirating bacteria-laden water, to people infected and killed by fungi carried on debris from the Joplin, Missouri, tornado, also in 2011.

But it can be hard to pinpoint when an infection or reaction is related to mold specifically, because the damage caused by disasters exposes victims to so many substances. “After these flooding events or hurricanes, there’s so much going on: Not only are you dealing with a house full of mold, but you’re ripping that house apart, so there’s drywall and dust and plaster and all kinds of things that you’re potentially inhaling,” says Tom Chiller, a physician and chief of the mycotic diseases branch of the Centers for Disease Control and Prevention. “It’s hard to tease out the effect of mold.”

Researchers thus face a conundrum: Their clinical instincts tell them people are at risk, but they have a dearth of data to prove it. Immune-compromised people are always at risk for mold and fungal infections; their diminished defenses render them unable to clear away the fungal spores that we all breathe in every day, leaving them vulnerable to organisms such as aspergillus and the ferocious mutant yeast Candida auris. The CDC estimates that more than 75,000 people are hospitalized annually for invasive fungal infections, and cost the health care system about $4.5 billion a year.

The ones most at risk are transplant patients who received donor organs or underwent leukemia treatment, and take immune system-suppressing drugs to sustain their recovery. Those people, researchers say, shouldn’t be anywhere near a moldy house, let alone working to remediate one, and should stay away from floodwaters. But in a survey of 103 immunosuppressed patients the CDC and several Houston hospitals conducted after Hurricane Harvey, half of them admitted they had gone back to clean out their flooded houses, and only two-fifths of that half said they had worn a protective respirator.

The CDC has been working with some of those hospitals on a more complex post-Harvey project, not yet published, which reviews medical records from one year before and after the hurricane to capture whether immune-suppressed people developed invasive fungal infections related to the storm. There’s no clear signal in the data, says Mitsuru Toda, an epidemiologist in the agency’s mycotic diseases branch: “In aggregate, we do see an increase after Hurricane Harvey in the number of people who had invasive mold infections, but some hospitals had a decrease, some hospitals had an increase, and the numbers are small.”

Complicating that finding, she adds, is that some mold and fungal infections have incubation periods long enough that symptoms might not have manifested during that post-storm year. Plus, Toda says, some physicians in Houston told the agency they preemptively put their most immune-suppressed patients on antifungal drugs—which protected those patients, but would have confounded any calculations of the hurricane’s effect on their health.

Ostrosky-Zeichner was one of those clinicians. “In theory, we should be seeing hordes of mold infections after major flooding events and hurricanes, but we’re not quite seeing that so far,” he says.

Researchers are also worried about the much larger proportion of the population, estimated to be up to 40 percent, who are prone to allergies and could react to mold and fungal growths in their houses—as well as about the rest of the population, who can develop new allergies after exposure. “For most people, the health effect that we see most often is respiratory,” says Felicia Rabito, an epidemiologist and associate professor at Tulane’s School of Public Health and Tropical Medicine. “A severe reaction would be like a breathing problem; a less severe reaction would be allergic-type symptoms. If you’re an asthmatic, though, and mold is a trigger, you can trigger an asthma attack, which is a very serious reaction.”