Epidemiologists believe that this strain originated as a mix of swine, human, and avian viruses. But why swine and not pig?

No good reason is known for the choice of a name for this illness, but a Google search turned up fewer than one million hits for pig flu and more than 69 million for swine flu (and just under three million hits for H1N1 flu, fewer than 400,000 for Mexican flu and not even 10,000 for North American flu).

No matter what name you use, here's the definition for swine flu or swine influenza from the Merriam-Webster's Medical Desk Dictionary:

an acute contagious febrile disease of swine that is marked by severe coughing and inflammation of the upper respiratory tract, that sometimes develops into bronchopneumonia but is rarely fatal, that is caused by infection with a subtype (H1N1 or sometimes H3N2) of the orthomyxovirus causing influenza A, and that is often complicated by infection with another microorganism (as the pseudorabies virus or a pneumonia-causing bacterium).