This year's flu season caught many experts off guard with both its sustained prevalence and its virulence. At its peak, there was a higher level of flu-like illnesses reported than any other year during the past eight years. Watch in the visual as it hits its peak around Week 18.
Why it matters: Public health officials try to capture this data when developing the next year's vaccines. And, of course, they want to find better ways to prevent severe flu seasons. There's a "Strategic Plan" to develop a universal vaccine to protect against a wider range of influenza viruses, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, tells Axios.