The flu (influenza) is an infection of the nose, throat and lungs caused by flu viruses (there are a number of different types). While the flu typically causes mild symptoms, it can be severe enough to cause hospitalization or even death. Flu is in the community every year, most commonly in the winter and early spring. Flu virus often changes every year and the changes may be significant.
When significant changes occur, this may result in many people getting infected. H1N1 is a new type of flu virus.