Enlarge / Computer illustration of Candida fungi. (credit: Kateryna Kon | Science Photo Library | Getty)

Fungi are an indispensable part of your microbiome, keeping the body’s host of microorganisms healthy as part of a system of checks and balances. But when you’re hit by an infection, fungi can be thrown out of equilibrium with other organisms inside you, leading to a more severe infection and other symptoms of illness.

For this reason, the pandemic immediately set off alarms for Iliyan Iliev, an immunologist at Weill Cornell Medical School. “We were thinking, the first thing that’s going to happen is people will start getting fungal co-infections,” he says. With the microbiome unbalanced, fungi might start running riot inside COVID patients, Iliev reasoned. His fears were soon realized.

In research published in Nature Immunology, he and his team discovered that in patients with severe COVID, certain strains of gut fungi—knocked off-kilter by the virus—set off a prolonged immune response that could last long after the initial infection. This response potentially led to some of the respiratory symptoms experienced by these patients. These results, Iliev says, point to the critical role of the gut microbiome in the human immune response and could lead to better disease treatments down the line.