A study published in the journal Nature on Monday confirmed that the blood of people born before 1920 carries antibodies to the 1918 strain, suggesting their immune systems remember a childhood infection.

The work by Dr. Yoshihiro Kawaoka also supports other studies that this new H1N1 strain does not stay in the nose and throat, as do most seasonal viruses.

"The H1N1 virus replicates significantly better in the lungs," Kawaoka said. Other studies have also shown it can cause gastrointestinal effects, and that it targets people not usually thought of as being at high risk.

"Obesity has been observed to be one of the risk factors for more severe reaction to H1N1" -- something never before seen, Kieny added. It is not clear if obese people may have undiagnosed health problems that make them susceptible, or if obesity in and of itself is a risk.

On Friday, a team at the U.S. Centers for Disease Control and Prevention and the University of Michigan reported that nine out of 10 patients treated in an intensive care unit there were obese. They also had unusual symptoms such as blood clots in the lungs and multiple organ failure.

None have recovered and three died.

The CDC estimates at least a million people are infected in the United States alone and clinics everywhere are advised not to test each and every patient, so keeping an accurate count of cases will be impossible. The United States has documented 211 deaths and WHO counted 429 early last week.

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