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 Allentown, Pa., hospital set up a tent outside its emergency room to deal with the feverish patients.

Flu season in the United States has struck early and, in many places, hard.

While flu normally doesn't blanket the country until late January or February, it is already widespread in 40 states, with about 30 reporting some major hot spots. On Thursday, 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 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 points to a moderate season, Schaffner and others say. 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.

Also, the flu's early arrival coincided with spikes in 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 call the flu may in fact be 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.

But flu is a big contributor to what's going on. "I'd say 75 percent," said Dan Surdam, head of the emergency department at Cheyenne Regional Medical Center, Wyoming's largest hospital.

Lehigh Valley Hospital-Cedar Crest in Allentown set up a tent this week for a steady stream of patients with flu symptoms. But so far, "what we're seeing is a typical flu season," said Terry Burger, director of infection control and prevention for the hospital.

On Wednesday, Boston declared a public health emergency, with the city's hospitals counting about 1,500 ER visits since December by people with flulike symptoms.