WASHINGTON, (Reuters) – The new H1N1 influenza virus  is now widespread, causing a range of illness, and U.S. health  officials said yesterday they fear it could mix with  drug-resistant versions of seasonal flu. 

So far the new strain of swine flu, which has killed seven  people in the United States and may have infected more than  100,000, responds well to treatment with antiviral drugs,  officials at the U.S. Centers for Disease Control and  Prevention say. 
 
But this year its cousin, a seasonal H1N1 strain, became  mostly resistant to the most commonly used antiviral — Roche  AG’s <ROG.VX> Tamiflu. GlaxoSmithKline’s <GSK.L> Relenza, an  inhaled drug, still works.  

The CDC said more seasonal flu viruses were making people  sick than would normally be expected for May, and the reasons  were not clear.  

“We think that it may be as much as half, or even more, of  the viruses that we are testing now … are the new H1N1 or  cannot be subtyped,” the CDC’s Dr. Anne Schuchat told reporters  in a telephone briefing.  

“The particular risk here … is that co-circulation of  this new virus together with the seasonal strains might put us  at risk for there to be a reassortment event.”  

Reassortment is the viral equivalent of sex — two viruses  can meet and swap entire stretches of their genetic material.  Flu viruses are especially prone to this and some pandemics  have emerged because of this genetic mixing. 
 
Flu viruses also mutate, by making mistakes when  replicating.