In the U.S., the last several years’ flu seasons have run with the weather as cases peaked in the cold days from December to February. This year, though, trips to doctors and hospitals for influenza symptoms are just starting to pick up, according to data from Athenahealth Inc., which provides software and services to physicians. About 3.3 percent of patients showed flu symptoms in the week that started March 6, the highest proportion so far this year.

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