(For full coverage of the flu outbreak, click [nFLU])

(Updates throughout with quotes, details from briefing)

WASHINGTON, May 12 (Reuters) - The United States now has 3,009 confirmed cases of the new H1N1 influenza across 45 states and Washington, D.C., the Centers for Disease Control and Prevention reported on Tuesday.

The outbreak of swine flu has been mostly mild in the United States, with three deaths, the CDC said in a statement.

CDC officials have said the virus is widespread across the country and testing is greatly underestimating the true number of cases.

The CDC's Dr. Anne Schuchat said health officials were especially worried because surveillance shows seasonal flu viruses are still circulating and could mix with the new H1N1 strain, either in people or in pigs.

"The working hypotheses for much of the influenza community right now is that this strain was circulating in pigs somewhere, and eventually reassorted and was able to infect humans easier," Schuchat told reporters in a telephone briefing.

"Unfortunately, reassortment happens. And this means that the viruses that we're seeing can exchange genetic material with other viruses that are circulating. This can happen in humans, in pigs, in birds," she said.

Not only are seasonal flu strains circulating, but so is the H5N1 avian influenza virus, which so far rarely infects people but which has killed 258 out of 423 people infected since 2003.

Schuchat said it was important to use antiviral drugs only as needed, because flu viruses easily evolve resistance to them. The seasonal strain of H1N1, a distant cousin of the swine flu, was widely resistant to Tamiflu, for instance, this year.

Influenza viruses are already mostly immune to the older flu drugs amantadine and rimantadine, as is the new H1N1 virus.

The World Health Organization and the CDC recommend using Tamiflu, known generically as oseltamivir and made by Roche AG <ROG.VX> under license from Gilead Sciences Inc <GILD.O>, and Relenza, known generically as zanamivir and made by GlaxoSmithKline <GSK.L> under license from Australia's Biota <BTA.AX>.

Schuchat said because of the risk of the viruses mixing, the CDC would especially stress its annual influenza-vaccination campaign for the 2009-2010 flu season.

(Reporting by Julie Steenhuysen and Maggie Fox; Editing by Bill Trott)

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