(NEXSTAR) – Influenza cases continue to mount with the virus reaching activity levels in the U.S. not seen for the last 15 years, according to the Centers from Disease Control and Prevention.

One indicator of flu activity is the percentage of doctor’s office visits driven by flu-like symptoms. Last week, that number was clearly higher than the peak of any winter flu season since 2009-2010, when a swine flu pandemic hit the nation, according to data posted Friday morning by the Centers for Disease Control and Prevention.

Of course, other viral infections can be mistaken for flu. But COVID-19 appears to be on the decline, according to hospital data and to CDC modeling projections. Available data also suggests another respiratory illness, RSV, has been fading nationally.

The flu has forced schools to shut down in some states. The Godley Independent School District, a 3,200-student system near Fort Worth, Texas, last week closed for three days after 650 students and 60 staff were out Tuesday.

Jeff Meador, a district spokesman, said the vast majority of illnesses there have been flu, plus some strep throat. He called it the worst flu season he could remember.

“I always like to reiterate that if you have the flu, stay home from work or school and isolate yourself from your family as much as possible until fever free for 24 hours (without fever reducing medications) and your symptoms are starting to improve,” Cleveland Clinic Infectious Disease Specialist Dr. Donald Dumford told Nexstar. “Even then, I’m a proponent of masking for the next several days after you are out of isolation.”

So far this season, the CDC estimates, there have been at least 24 million flu illnesses, 310,000 hospitalizations and 13,000 deaths — including at least 57 children. Traditionally, flu season peaks around February.

Overall, 43 states reported high or very high flu activity last week. Flu was most intense in the South, Southwest and western states.

Cleveland Clinic doctors recommend doing the following to help prevent the spread of flu:

• Proper handwashing.
• Avoiding touching your eyes, nose and mouth.
• Avoiding people who are sick.
• Wearing masks in situations when you can’t avoid people who are sick, such as caring for a sick child.

The Associated Press contributed to this report.