The flu has started spreading earlier than usual in some parts of the country, prompting concerns that cases could soon erupt as millions of Americans travel and gather for Thanksgiving and other upcoming holidays.

The warning signs come as a form of the virus, called H3N2 subclade K, has set off massive outbreaks in Canada and the U.K.

Dr. Cameron Wolfe, an infectious disease specialist at Duke University School of Medicine in Durham, North Carolina, said he’s noted a sharp increase in flu cases over the past few weeks.

“Typically, I might see one or two cases a week at this point,” Wolfe said. “At the moment, we’ve had two, three, four per day coming in.”

Alicia Budd, head of the Centers for Disease Control and Prevention’s domestic influenza surveillance team, said: “We certainl

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