In 1997, doctors from Cleveland reported on a wave of infants in the area who had come down with severe lung bleeding—an illness that might have been caused by so-called toxic black mold.

In the winter of 1994, they wrote, at least 10 children developed these symptoms; one nine-week-old infant tragically died as a result. An investigation by public health officials identified another 11 similar cases between 1995 and 1996, with two deaths.

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There was no clear biological cause for the cases, like a virus or bacteria, but the investigators noticed that the affected children all lived in water-damaged homes. And these homes were much more likely to contain a mold known as Stachybotrys chartarum. While the doctors were cautious in speculating about a possible link between the mold and the children, their work helped establish the now-popular idea that S. chartarum is especially dangerous.

But they were wrong, according to a later CDC investigation, published in March 2000. Today, the CDC doesn’t believe this mold is particularly dangerous. But public fear of toxic “black mold” has persisted.

Nonetheless, molds and their byproducts can indeed be harmful, and even deadly, especially for people with weakened immune systems. A common mold called Aspergillus, when inhaled by particularly susceptible people, can infect the lungs and spread throughout the body. Other molds that grow in our foods can produce toxins that sicken us quickly or increase our risk of cancer over time.

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In our latest video, above, we explore the various ways molds pose a danger to people, as well as some of their helpful qualities (we can thank mold for the life-saving antibiotic penicillin, of course). The truth is, you’re likely breathing in mold spores at this very moment—but if your immune system does its job, you have little to worry about.