The map of flu activity across the United States continues to darken, both figuratively and literally, as case numbers spike upward in most states, according to the Centers for Disease Control and Prevention (CDC).

Outpatient health care visits for flu-like illnesses are at their highest levels in 15 years, according to data updated Friday by the CDC.

There have been at least 24 million illnesses, 310,000 hospitalizations and 13,000 deaths from flu so far this season, according to the CDC. The agency said in its weekly flu surveillance report that seasonal influenza activity continues to increase across the country.