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

Flu activity remains “very high” in most states, the Centers for Disease Control said this week, as illnesses, emergency room visits and hospitalizations all continue to trend upward.

Bird flu cases are still rising in the U.S. as the virus continues to devastate poultry farms. More than 145 million chickens, ducks, turkeys and other fowl have been slaughtered across the United ...