CDC flu chief Dr. Nancy Cox said the good news is "we do not see the markers for virulence that were seen in the 1918 virus."
Nor does swine flu virus have the virulence traits found in the H5N1 strain of bird flu seen in recent years in Asia and other parts of the world, she said.
"However we know that there is a great deal that we do not understand about the virulence of the 1918 virus or other influenza viruses," that caused serious illnesses, she said. "So we are continuing to learn."
Another CDC official, Dr. Anne Schuchat, said preliminary studies suggest that in U.S. households with an infected person, about a quarter of other family members are getting sick as well.
Generally, for seasonal flu, between 5 percent and 20 percent of those exposed to the virus get sick, depending on the setting.
In some pandemics, the rate has been as high as 35 percent, Cox said.
She noted the CDC has entered the gene information for the new virus into databases that are publicly available.
"A lot of researchers around the world can begin to look at those gene sequences as well, in case they see something we haven't already seen," Cox added.
The global flu epidemic early last century was possibly the deadliest outbreak of all time. The virus was an H1N1 strain - different from the H1N1 strain involved in the current outbreak - and struck mostly healthy young adults. Experts estimate it killed about 40 to 50 million people worldwide.
On the Net:
For facts about influenza, and more information about swine flu, please visit the Health Department and CDC websites. Some specific resources:
From New York City Health Department
Facts about flu
From Centers for Disease Control and Prevention
General information about swine flu
Swine Flu Case Definitions
Swine Flu Infection Control and Patient Care
Preventing the Flu
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