Influenza levels in the US are the highest they've been in 15 years amid winter weather and a second wave of the virus.

The U.S. winter virus season is in full force, and by one measure is the most intense in 15 years.

which is the highest seen since the 2009-10 flu season, during the swine flu pandemic, data shows. During the week ending Feb. 1, 8% of emergency department visits were due to flu compared with 3.2% ...