An influenza is a contagious viral infection. Influenzas tend to spread during the winter, and they're more commonly known as the flu.
If you've ever wondered why this word looks so much like influence, that’s because both come from the Italian influentia. The original idea was a flowing in — of ethereal fluid from the heavens, thus affecting human destiny. We know now influenza is caused by viruses, not heavenly fluid. When you have an influenza, you're likely to vomit, have diarrhea, be exhausted, and feel terrible overall. One way to prevent influenza is by getting a flu shot.