Study Details Swine Flu Transmiss...(is also director of the Centers for Dise...),Health

"What that means is, three days later, you've got an additional half a person infected. In three days, each of those new people have infected an additional half person, and it's like compound interest. It's the same calculation that lets you grow $1,000 into a $1 million 20 years later."

But health officials stress that the H1N1 swine flu produces relatively mild infections, much like the annual seasonal flu, and patients recover quickly. And some people, mostly older ones, seem to have some immunity to the virus.

However, doctors around the world are reporting a very severe form of the disease in young and otherwise healthy people. "In these patients, the virus directly infects the lung, causing severe respiratory failure," the WHO said. So, countries should anticipate a growing demand for treatment in intensive care units as they prepare for a second wave of the pandemic, the agency said, the Associated Press reported.

Dr. Peter Gross, chief medical officer at Hackensack University Medical Center in New Jersey, sees no reason for mass panic. "They've proven that the transmissibility is comparable to the seasonal flu and less than the horrendous 1918 pandemic," he said. Also key, he said, is that "the mortality is no worse than the seasonal flu and, if anything, might be slightly less."

Still, he and others agree that the potential for infection is significant.

"If each person were infecting less than one person, it would eventually die out on its own. If it was two people, the outbreak would cascade. If it was 10, it would be an explosive epidemic," said Dr. Dean Blumberg, associate professor of pediatric infectious diseases at University of California, Davis Children's Hospital. "The number they've come up with here is similar to what others have found, in the range of 1.3 to 1.4. The seasonal
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