Influenza is a disease caused and spread by a virus. Influenza viruses belong to the Orthomixovirus family and can infect humans and some animals. The characteristics of influenza viruses mutate often and easily, which is why each year a new strain of influenza virus circulates.

There are three types of influenza viruses, called A, B and C.:

• Virus A is responsible for most influenza epidemics each year. It is the most dangerous one and most easily mutates.

• Virus B is the less common form. It causes less severe forms of influenza. Generally the influenza epidemics due to this virus occur every five years.

• Virus C causes infections that may lack symptoms, are very light and resemble heavy versions of common colds. They occur frequently in children. 

Influenza viruses are mainly of Asian origin. In these geographic areas, environmental conditions favor the development and the mutation of the virus. The influenza virus stems from birds, which transmit them to pigs which, in turn, pass them on to man.

Because influenza viruses constantly evolve, immunity is impossible. Our immune systems recognize a virus if we have already been infected once by the same type of virus, but if the virus changes, recognition is no longer possible. Thus, every year new vaccines must be developed.