H1N1 flu, also known as swine flu, is a type of influenza A virus that causes upper and lower respiratory tract infections in humans and pigs. The virus is called swine flu because it is similar to a flu virus that affects pigs. The 2009 H1N1 virus was a new combination of influenza viruses that infect pigs, birds, and humans, and it spread through airborne droplets from human to human, and potentially, through human contact with inanimate objects contaminated with the virus and transferred to the eyes or nose. The World Health Organization declared the H1N1 flu to be a pandemic in 2009, and that year the virus caused an estimated 284,400 deaths worldwide. Most people with the flu get better on their own, but flu and its complications can be deadly, especially for people at high risk. The symptoms of flu caused by H1N1 are similar to those of other flu viruses, including fever, cough, sore throat, body aches, headache, chills, and fatigue. The seasonal flu vaccine can help protect against the H1N1 flu and other seasonal flu viruses.