Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza virus that regularly causes outbreaks of influenza in pigs. Like all influenza viruses, swine flu viruses change constantly. Pigs can be infected by avian influenza and human influenza viruses as well as swine influenza viruses. At this time, there are four main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. However, most of the recently isolated influenza viruses from pigs have been H1N1 viruses. The H1N1 swine flu viruses are antigenically very different from human influenza A H1N1 viruses and, therefore, vaccines for human seasonal flu would not provide protection from H1N1 swine flu viruses.
Since this is therefore a novel virus for human immune systems, we don’t have much natural immunity, thus allowing the virus to spread quickly. This is why there is worldwide concern over the possibility of this becoming a pandemic (A pandemic (from Greek pan all + demos people) is an epidemic of infectious disease that spreads through populations across a large region; for instance a continent, or even worldwide.( http://en.wikipedia.org/wiki/Pandemic).