Influenza is a common respiratory virus that is categorized into four types: A, B, C and D. Type C only causes mild infections and type D is not believed to infect humans.

Influenza epidemics, caused by influenza A or B viruses, result in acute respiratory infection. They kill half a million people worldwide every year. These viruses can also wreak havoc on animals ...

This is the worst flu season in 15 years. Here's why—and how to protect yourself. The number of influenza cases is surging across the United States, putting significant strain on many hospitals.