During COVID-19 infection, a fungal marker called beta-D-glucan (BDG) in the blood can predict patient survival even without a diagnosed fungal infection. Researchers found that COVID-19 patients with high BDG levels had a 91% death rate if not treated with antifungal drugs, but this dropped to 50% when antifungal therapy was given. The high BDG levels trigger a strong inflammatory response in the body that worsens disease severity, making BDG a valuable warning sign for doctors treating critically ill COVID-19 patients.