Beyond those groups we know are susceptible to severe illness – the old, frail or generally unwell – it has been unclear why the disease hits some healthy people hard and presents as no more than a mild cold in others.

One hypothesis presented in 2022 suggested a person's blood type plays a crucial role in determining the severity of COVID. But another earlier study pointed to gut bacteria differences influencing both disease severity and long COVID risk.

Hundreds of recent studies have explored the relationship between our bacterial gut microbiome and COVID but few have focused on the less popular fungal populations within us. Known as the mycobiome, it was only a decade ago that scientists discovered our guts carry a resident population of fungi. Knowing this fungal population has been associated with inflammatory changes in the body, a new study led by researchers from Weill Cornell Medicine set out to investigate whether there was a direct link between the mycobiome and severe COVID.

The first stage in the research involved studying blood samples from 91 COVID patients (25 with mild or moderate illness and 66 with severe disease). Another 36 blood samples were taken from healthy subjects. It was quickly discovered that those patients experiencing severe COVID had high volumes of immune antibodies that were tuned to target specific gut fungal populations.

Looking at fecal samples from a small subset of patients one fungus species in particular seemed to stand out in the sickest subjects, Candida albicans. The researchers could draw a distinct correlation between increasing levels of Candida and COVID disease severity.

To further home in on this compelling correlation the researchers colonized a collection of healthy mice with Candida fungus isolated from human COVID patients. The mice were then infected with SARS-CoV-2 and the researchers noted the animals experienced a significantly greater inflammatory response to the virus compared to a group without the fungus. Even more compelling, the heightened inflammatory response to SARS-CoV-2 in the mice could be tamped down by administering antifungal drugs.