Many people think of mould as a problem that stains bathroom walls or appears after a flood.
However, new research suggests that some types of mould can cause severe infections inside the body and that these illnesses may be more common than previously recognised.
The research was published by the U.S. Centers for Disease Control and Prevention. Scientists wanted to answer a basic question that had never been carefully studied in the United States: how many patients are actually treated for invasive mould disease?
Doctors do not have to report these infections nationally, making it difficult to understand their true impact. To solve this problem, researchers examined patient records from four Atlanta hospitals and their associated clinics between 2020 and 2024.
The team initially identified nearly one thousand possible patients. After reviewing medical information in detail, they confirmed around 450 genuine cases of invasive mould disease. This careful approach reduced the chance of counting people who had only been exposed to mould without developing a serious infection.
Mould spreads by releasing tiny spores into the air. Healthy people usually breathe these spores in without noticing because the body’s immune system destroys them. For people whose immune systems are weakened, however, the spores may grow deep inside the lungs or other organs.
The study found that many affected patients had cancer, other immune disorders or had become vulnerable after severe COVID-19. These groups already face many health challenges, making invasive fungal infections especially dangerous.
One of the most worrying findings was the high death rate. About one in three patients died during their hospital stay. The study cannot prove mould alone caused every death because many patients were already critically ill, but it clearly shows these infections are linked with serious outcomes.
The researchers estimated that hospitals should expect several invasive mould cases each year for every one hundred hospital beds. Academic medical centres reported more cases because they often care for patients with the most complex diseases.
Experts believe the problem could become more important in coming decades. Extreme weather events linked with climate change create damp environments where mould grows easily. At the same time, medical advances are helping more people survive cancer and organ transplantation, increasing the number of people living with weakened immune systems.
The authors emphasised that the findings should not be assumed to represent every part of the country. More studies in different climates and healthcare systems are needed before national estimates can be made.
This research provides an important first step toward understanding a neglected health problem. The study’s strengths include real hospital data collected over five years and careful case verification. Its main limitation is that it focused on a single metropolitan area.
Future nationwide studies could help guide infection control policies, improve hospital preparedness and identify patients who would benefit most from early diagnosis and treatment.
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Source: U.S. Centers for Disease Control and Prevention (CDC).
