NEW YORK — From the Rocky Mountains to New England, hospitals are swamped with people with flu symptoms. Some medical centers are turning away visitors or making them wear face masks, and one Pennsylvania hospital set up a tent outside its emergency room to deal with the feverish patients.

Flu season in the U.S. has struck early and, in many places, hard.

While flu normally doesn’t blanket the country until late January or February, it already is widespread in more than 40 states, with about 30 of them reporting some major hotspots. On Thursday, health officials blamed the flu for the deaths of 20 children so far.

Whether this will be considered a bad season by the time it has run its course in the spring remains to be seen.

“Those of us with gray hair have seen worse,” said Dr. William Schaffner, a flu expert at Vanderbilt University in Nashville.

The evidence so far points to a moderate season, Schaffner and others said. It looks bad in part because last year was unusually mild and because the main strain of influenza circulating this year tends to make people sicker and really lay them low.

David Smythe of New York City saw it happen to his 50-year-old girlfriend, who has been knocked out for about two weeks. “She’s been in bed. She can’t even get up,” he said.

Also, the flu’s early arrival coincided with spikes in a variety of other viruses, including a childhood malady that mimics flu and a new norovirus that causes vomiting and diarrhea, or what is commonly known as “stomach flu.”

So what people are calling the flu might be, in fact, something else.

“There may be more of an overlap than we normally see,” said Dr. Joseph Bresee, who tracks the flu for the Centers for Disease Control and Prevention.

Most people don’t undergo lab tests to confirm flu, and the symptoms are so similar that it can be hard to distinguish flu from other viruses, or even a cold.

Over the holidays, 250 people were sickened at a Mormon missionary training center in Utah, but the culprit turned out to be a norovirus, not the flu.