May 15, 2009 -- More than 100,000 Americans probably have the flu -- and at least half of these cases are H1N1 swine flu, a CDC expert estimates.

The comment came from Daniel Jernigan, MD, PhD, deputy director of the CDC's influenza division, during a news conference.

Jernigan noted that the 4,700 confirmed or probable cases of swine flu reported to CDC represent a gross underestimate. When asked how many actual cases there were, Jernigan noted that 7% to 10% of the U.S. population -- up to 30 million people -- get the seasonal flu each year.

"So with the amount of activity we are seeing now, it is a little hard to know what that means in terms of making an estimate now of the total number of people with flu out in the community," Jernigan said. "But if I had to make an estimate, I would say ... probably upwards of maybe 100,000."

The CDC's most recent data, for the week ending May 9, shows that about half of Americans with confirmed flu had the H1N1 swine flu. If Jernigan's off-the-cuff estimate is correct, more than 50,000 people in the U.S. have the new flu.

At a time when flu season should be ending or over, the CDC's flu season indicators are going up instead of down. As of May 9, 22 states had widespread or regional flu.

Meanwhile, Arizona's Maricopa County -- home to Phoenix -- reported that a woman in her late 40s died from complications of the H1N1 swine flu. She is the nation's fourth H1N1 swine flu fatality. The woman had an underlying lung disease, according to the Maricopa County public health department.

Despite the death, the CDC's marker for flu severity -- deaths from pneumonia or flu -- did not increase beyond normal levels for this time of year. This so-called "epidemic threshold" is 7.4% of all deaths; for the week ending May 9, flu/pneumonia deaths were 7.2% of all deaths.

One of the most alarming signs of a flu pandemic is a lot of severe illness in people who don't usually suffer severe flu cases -- older children and young adults.

Most of the 173 people hospitalized in the U.S. with H1N1 swine flu have been between 5-24 years old.

"At this point, we're not seeing the seriousness of illness in the United States that was initially reported in Mexico, but this certainly does not mean that the outbreak is over," Jernigan said. "The H1N1 virus is not going away. We know that the outbreak is not localized but is spreading and appears to be expanding throughout the United States.