Cases of this type of influenza H1 virus appeared to pick up more recently, with eight of the 11 cases being reported by the CDC after June 2007, the team noted. They also said that these were only those cases spotted via the CDC's routine "passive" flu surveillance systems -- additional but unidentified cases may have occurred.
"It's not surprising that they have reports of cases going back further," said Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City. "There may have been more than 11 sporadic cases but it probably reached a critical mass where it could spread or it reassorted. Reservoirs of virus found in animals do come to humans."
But unlike those early cases, the H1N1 virus is now hopping easily from human to human.
"What's unusual about this particular case is that it's able to establish sustained [human-to-human] transmission," Shaw said.
"We don't yet know why this one is passing so much better than the other ones," added John Quarles, head and professor of microbial and molecular pathogenesis at Texas A&M Health Science Center College of Medicine. "This is actually still pretty early in the business." Quarles repeated a message appearing in editorial in the journal: This is not the last new influenza virus we will see.
According to Shaw, the new swine flu strain and the circulating seasonal H1N1 influenza strains can all be traced back to the virus that sparked the 1918 pandemic, which killed millions worldwide. "All have been evolving along separate track
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