This background review produced by Massachusetts General examines why bacterial superinfection is a driver of mortality in viral pneumonia such as influenza and COVID-19. It describes why doctors are concerned about bacterial infections if COVID-19 is a viral illness.
Basically, the longer a patient stays in a hospital setting, the more risk they have in getting a bacterial or fungal infection in addition to their COVID-19.
The secondary infections observed in COVID-19 patients are a consequence of viral pneumonia but also prolonged hospital exposure and critical illness. Nosocomial infections related to long hospital course impact severity of COVID-19 and increase mortality.