The patient’s own body makes the secretions. It’s a combination of leakage from damaged cells in the lungs caused by the virus, and the natural immune response.

The patient’s own body makes the secretions. It’s a combination of leakage from damaged cells in the lungs caused by the virus, and the natural immune response. Since the infection is inside the lungs, producing mucus is the body’s way to try to encapsulate and expel contaminants, and also provides a liquid medium for antibodies and white blood cells to circulate in. With covid-19 and flu, you generally don’t die from the effects of the virus, you die from the side-effects of your own body’s immune response.