NASHVILLE, Tenn. (WZTV) — The CDC says influenza, commonly called the flu, will continue to spread for several more weeks and Tennessee is among states of high alert.

More than a dozen middle Tennessee school districts are heading back to school Tuesday after illnesses, including the flu, closed classes for a week at some locations.

Flu viruses can be active as late as May, health officials said, and they are reminding people that it's not too late to get vaccinated.

Seniors are especially vulnerable, with a higher rate of flu hospitalizations and death.