Swine Flu is a strain of the influenza virus primarily associated with pigs, and known as swine influenza virus or SIV.
The 2009 swine flu outbreak, largely believed to have originated in Mexico, is believed by health officials to be a new strain of influenza A virus subtype H1N1. The source, though unconfirmed, is likewise thought to be one strain of human influenza virus, one strain of avian influenza virus, and two separate strains of swine influenza virus.
The 2009 outbreak has created enough concern to close schools, and given cause for the World Health Organization (WHO) to issue a Level Four Warning, indicating significant human to human transmission, and considered one level short of a pandemic.
Reports of the outbreak have been confirmed worldwide, including cases in Mexico, Canada, Spain, Scotland, and at least ten states in the U.S. — where President Obama has acknowledged cause for concern. Obama urged citizens who are sick to stay at home, but has also said there is no cause for panic.