Pneumonia
“Influenza is dangerous, but the flu itself almost never kills anyone,” says Dr. Cloonan. What does is a secondary infection of bacterial pneumonia—it can accompany the flu. A common scenario: Someone who has the flu starts to feel better, but then a second wave hits. “That’s bacterial pneumonia,” says Dr. Cloonan. “If the fever goes away and comes back, you should be concerned that pneumonia happened after the flu has run its course.”