How often do humans become infected with H1N1 flu?
In the past, the federal Centers for Disease Control and Prevention (CDC) received very few reports of H1N1 flu infecting humans in the United States. Until the recent outbreak in Mexico and the apparently related ones in this country, only 12 cases of human H1N1 flu had been reported in the United States between December 2005 and February 2009. The WHO notes, however, that mild cases or ones without typical symptoms could have escaped detection, so the true extent of H1N1 flu among humans is unknown.
