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The flu's incubation period is believed to be between one and seven days. What's most puzzling is how -- and how fast -- the bug jumps from person to person, The Wall Street Journal reported Monday.

The H1N1 (swine flu) spread between people in the United States and Mexico, the virus's epicenter, suggests it transmits efficiently, scientists said, adding they're trying to collect better data.

But the biggest challenge facing scientists is why H1N1 appears to have killed many people in Mexico, but not elsewhere. The only death beyond Mexico's borders was in the United States -- that of a child from Mexico visiting Texas.

"You need to look at anti-bodies in people -- how many got infected, how many got serious or mild disease, and how many died", said Albert Osterhaus, a virologist at Erasmus University in the Netherlands who is testing the virus in ferrets.

To determine virulence in a new bug, scientists try to learn if it shares pathogenic markers with older, deadly viruses. Osterhaus' lab is comparing markers on the current H1N1 bug to other viruses that were deadly in people, such as the 1918 strain.

"When the Spanish flu started in 1918 it was a relatively wimpy virus. Then it heated up", killing 40 million people, Osterhaus told the Journal. "I wouldn't be too comfortable that this swine flu virus isn't hot yet."

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