From the National Institutes of Health in 2008:

“The majority of deaths during the influenza pandemic of 1918-1919 were not caused by the influenza virus acting alone, report researchers from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. Instead, most victims succumbed to bacterial pneumonia following influenza virus infection. The pneumonia was caused when bacteria that normally inhabit the nose and throat invaded the lungs along a pathway created when the virus destroyed the cells that line the bronchial tubes and lungs.”

At the time, the NIAID was headed up by some guy named Fauci.