If a third of people wind up catching swine flu, 15 states could run out of hospital beds around the time the outbreak peaks, a new report warns Thursday.

The nonprofit Trust for America's Health estimates the number of people hospitalized could range from a high of 168,000 in California to just under 2,500 in Wyoming.

The public health advocacy group used government flu computer models to study how quickly hospitals would fill up during a mild pandemic, like the kind the swine flu -- what doctors prefer to call the 2009 H1N1 strain -- is shaping up to be. It based its estimates on the mild 1968 pandemic, suggesting up to 35 percent of the population could fall ill.

Even though only a fraction would be sick enough to be hospitalized, health officials are bracing: When H1N1 first appeared in the spring, more than 44,000 people visited emergency rooms in hard-hit New York City, the report noted. Just sorting out which patients are sick enough to be admitted from the vast majority who need to go home is a big job. And hospital capacity varies widely.

By the outbreak's peak, the new report suggests Delaware and Connecticut hospitals would fill up soonest. Also on that list: Arizona, California, Hawaii, Maryland, Massachusetts, Nevada, New Jersey, New York, Oregon, Rhode Island, Vermont, Virginia and Washington.

To deal with overcrowding from emergencies, hospitals are supposed to have "surge" plans -- when they would postpone elective surgeries to free up beds, for instance, and when they might even need to call in government help for mobile hospital units.