The Centers for Disease Control and Prevention's top flu specialist has privately informed health care officials that the H1N1 virus is becoming a more serious threat, CBSNews.com has learned.
"We're now certainly starting to pick up more serious cases," Dr. Joe Bresee, chief of CDC's epidemiology and prevention branch of the influenza division, told health care providers on Thursday.
Bresee said on a private conference call that the situation is now "changing a bit," adding that H1N1's incubation period is between one and four days, and its infectious period is similar to seasonal flu at two to seven days.
CDC spokesman Thomas Skinner told CBSNews.com that "we're still learning about" the H1N1 virus -- also known as swine flu -- and declined to comment further on his colleague's remarks.
Bresee's comments come as more evidence of the new influenza strain popped up around the country, with a dozen states reporting cases and nearly 300 schools closing. Meanwhile, the CDC is racing to develop a vaccine.
One puzzling question, of course, is why the H1N1 cases in Mexico seem to be resulting in more deaths than the ones in the United States.
Even though the CDC has confirmed 109 cases domestically, plus a few more reported by state health officials, there has been only a single known death -- a toddler visiting from Mexico. The World Health Organization reports 97 confirmed cases of infection in Mexico and seven deaths.
By comparison, the annual flu season kills about 41,000 Americans a year, and, according to the WHO, results in between "250,000 and 500,000 deaths every year around the world."