Influenza A, respiratory virus and the main cause of the flu, is one of the three types of Influenza. There are also types B and C that cause less severe diseases. Originated from aquatic birds where it infects the digestive system and normally does not cause symptoms, it is an RNA virus with 8 genes surrounded by a capsule and a lipid membrane that carries the infected cell. Different from many other viruses, Influenza is able to change its proteins sufficiently to escape from our immune system and re-infect another person, circulating annually and causing serious health problems. This is worsened by the virusÆ capacity of mixing itsgenes with genes of swine and aviary origin, events that sometimes resulted in serious pandemics during the last century.