Just found this information on Wikipedia's article on the outbreak:
The U.S. Centers for Disease Control and the World Health Organization are concerned that this outbreak may become a pandemic, because:
* The virus is a new strain of influenza, from which human populations have not been vaccinated or naturally immunized.
* The virus has produced severe disease in Mexico, and some deaths. Furthermore, in Mexico (but not in the United States) the illness has primarily struck young, healthy adults, much like the deadly Spanish Flu of 1918. This is unlike most influenza strains which produce the worst symptoms in young children, elderly adults, and others with weaker immune systems.
* The virus appears to infect by human-to-human transmission. Investigations of infected patients indicate no direct contact with swine, such as at a farm or agricultural fair. In contrast, for example, disease transmission in the last severe human outbreak of influenza, the bird flu that peaked in 2006, was determined to be almost entirely from direct contact between humans and birds.
* The virus has been detected in multiple regions within Mexico and multiple areas in the United States.