But 20 to 40 percent do not fit that profile. Some may have gotten medical care too late. Some may have had underlying conditions not known to them or their doctors or factors, such as a genetic predisposition, that science has yet to discover. Others may simply have lost out in the roll of the dice, like the nonsmoker who dies of lung cancer.
As a society, we tend to think everything has a clear cause, said epidemiologist Laurie Kamimoto, who leads the U.S. Centers for Disease Control and Prevention's surveillance of influenza hospitalization.
"I don't think people cozy up to the idea that it [may be] just bad luck," she said.
Even as more people die from swine flu, new cases have dropped dramatically, a concern to experts who fear that people will skip vaccination, making them vulnerable to a return wave.
Scientists worldwide are trying to identify who is most at risk from novel H1N1 influenza. Children were identified early on. The biggest risk factors for seasonal flu - pulmonary, heart, and metabolic diseases (such as diabetes), and immune systems that are suppressed (by drugs for cancer and diseases such as HIV) - do raise the risk of complications from swine flu.
Pregnancy, another known risk, appears to be an even greater factor in this pandemic. Hypertension, a history of smoking, and obesity also may make people more vulnerable.
Morbidly obese people (body mass index of 40 and above), currently 5 percent of the U.S. population, were hospitalized at five times the expected rate, Kamimoto and colleagues reported in the New England Journal of Medicine last month. Most had other chronic conditions, however, so the reasons are unclear.
