In children, other much rarer chronic conditions, such as sickle cell anemia, cerebral palsy and muscular dystrophy, are also predisposing patients to life-threatening bouts of the virus, federal health officials said.

Epidemiologists at the Centers for Disease Control and Prevention studied the experience of about 1,400 people older than 18, and 500 children who had been hospitalized in 10 states since the new influenza strain emerged in April. The states were California, Colorado, Connecticut, Georgia, Maryland, Minnesota, New Mexico, New York, Oregon and Tennessee.

Among the hospitalized adults, 26 percent had asthma; 10 percent had diabetes; 8 percent had a chronic lung disease such as emphysema; 7.6 percent were immunosuppressed from cancer, HIV infection or another ailment; and 6.1 percent were pregnant. (Heart disease was also common, although CDC officials could not immediately say what fraction of patients had it.)

In all, 45 percent of adults ill enough to be admitted to the hospital had a preexisting condition.

CDC epidemiologists do not know what fraction of the rest were obese -- a newly recognized risk factor for severe or fatal flu. An earlier analysis of 227 patients found that 15 percent were obese, and 8 percent were morbidly obese.

Among children hospitalized with flu, asthma and neurological or muscular diseases were the most common underlying conditions, followed by blood disorders, especially sickle cell. About 6 percent of children had a blood disorder; percentages for the other conditions were not available.

Anne Schuchat, a physician who directs the CDC's center for immunization and respiratory diseases, said there are two reasons why pregnant women are at an unusually high risk for flu complications.

Pregnancy is a state of controlled immunosuppression that keeps the mother from rejecting the fetus as "non-self" but may also reduce her capacity to resist infection. The enlarging womb also compresses the lungs, which can be a problem.

"It's harder to take a deep breath and it's harder to fight off a lung infection, especially in the later stages of pregnancy," Schuchat said in a telephone briefing for reporters. "Both of those are part of the story here, and of course the really difficult complications in pregnancy have been one of the features of the pandemic."

In another matter, the CDC official said the first supplies of the injectable H1N1 vaccine are being shipped to state and city health departments now.

Including both injectable and nasal spray vaccine (which can be used only by healthy people ages 2 to 49 years), 9.8 million doses were available and 5.8 million doses had been shipped as of Monday.

The federal government bought 250 million doses of H1N1 vaccine and expects there will be enough for anyone who wants it. In June, Congress appropriated $7.65 billion for response to the pandemic, and at least $6.39 billion of that already has been spent or committed.