Each year nearly 300,000 U.S. patients get serious cases of pneumonia and sepsis  -- bloodstream infections -- during their hospital stays. Almost 50,000 of them die.

That's the bottom line of a new study that its authors say is the first truly national look at the stubborn problem of hospital-acquired infections. It's based on 69 million hospital cases in 40 states.

These potentially avoidable infections result in longer hospital stays, costing more than $8 billion a year.

For all the attention devoted to the problem in recent years, study author Ramanan Laxminarayan says no study has ever been able make a credible estimate of deaths from infections that occur in the hospital. Earlier studies have been too small, he says, and could not distinguish between infections acquired in the hospital and those that patients had when they were admitted.

Authors of the new study, in the Archives of Internal Medicine, say they've gotten around this problem by looking, in part, at infections that occur among elective surgery patients. They're not likely to have been infected prior to admission, Laxminarayan says, or they wouldn't have had the surgery.

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