2009 H1N1 Influenza (sometimes called "swine flu") is a new influenza virus causing illness in people. This virus is spreading from person-to-person worldwide, in much the same way that seasonal influenza viruses spread. This season, there is a seasonal flu vaccine to protect against seasonal flu viruses and a 2009 H1N1 vaccine to protect against the 2009 H1N1 influenza virus. A flu vaccine is the first and most important step in protecting against flu infection.