The U.S. winter virus season is in full force, and by one measure is the most intense in about 15 years. One indicator of flu activity is the percentage of doctor's office visits driven by flu-like symptoms. According to data posted Friday, that level last week was higher than the peak of any winter flu season since 2009-2010. So far this season, the CDC estimates, there have been 13,000 flu deaths — including at least 57 children. The flu has forced schools to shut down in some states. One Texas district closed for three days because 650 students and 60 staff were out one day last week.