Flu season is underway, according to the Centers for Disease Control. The CDC monitors and analyzes several indicators that signal the start of the flu season. Those criteria were met in the week ending Dec. 15.

The CDC is reporting elevated levels of influenza activity nationally, and is expecting that level will continue for several weeks. State by state reports are helping map the geographical spread of the flu, but the CDC notes it's too soon to assess this season's flu severity.

Flu seasons over the past five years have lasted an average of 16 weeks, with a range of 11 weeks to 20 weeks. The CDC recommends that anyone who has not yet gotten a flu vaccine this season should get one now. It takes about two weeks for the vaccine to fully kick in. Flu can be serious, the CDC reminds us, even for otherwise healthy people.

John Lynch, medical director of infection control at Harborview Medical Center in Seattle, talks about flu prevention in the following video: