Influenza (the flu) is a contagious respiratory illness caused by several types and subtypes of viruses. One or more virus strains cause a surge in flu nearly every year, and can cause mild to severe illness, and at times can lead to death. The viruses undergo continuous genetic changes, so people don’t achieve permanent immunity. Vaccines must be updated every year to combat the anticipated predominant strains.
Flu activity most commonly peaks in the U.S. in January or February.
141031-M-BN069-010 Photo by Cpl. J. R. Heins