Influenza is a contagious respiratory illness caused by influenza viruses. In essence, viruses are collections of genetic information directed toward one end: their own replication. Viruses cannot replicate without infecting a living host. In otherwise healthy people, influenza infection usually produces tolerable symptoms and is efficiently cleared from the body by the immune system. Flu viruses though, have evolved to ensure the constant generation of new types to evade the immune system, and occasionally these new mutated viruses are especially efficient at spreading throughout the population. Three worldwide (pandemic) outbreaks of influenza occurred in the 20th century. On June 11, 2009 the World Health Organization declared a new pandemic resulting from a novel strain of Influenza A (H1N1).