General information

Influenza (flu) is a common and contagious infectious disease which is prevalent the world over and can affect anyone at any age.

Flu is caused by influenza viruses, which are classed into three types: A, B and C. These flu viruses are constantly mutating, which is why new strains of flu appear. They are classed depending on their geographic origin and year of appearance. Each strain therefore avoids the pre-existing immune response of the host (either human or animal) and can therefore affect a host who has previously been infected with a different strain.

• The influenza A virus is the most common. It regularly infects a large number of people during the winter months and causes a considerable number of deaths around the world.
• The influenza B virus causes serious respiratory conditions.
• The influenza C virus is not considered serious as it leads to very benign respiratory illnesses, among others.

These three types of virus affect humans, with the exception of type A, the only one to affect both humans and some kinds of animals (birds, pigs, horses, etc.). The influenza A virus is classed into 144 combinations of subtypes (16x9): 16 subtypes H and 9 subtypes N.

The two letters H and N designate the virus' surface antigens:

• subtypes H (hemagglutinin) are the most important from an epidemiological point of view, as they determine the virus' ability to attach to cells and enter them to then multiply themselves;
• subtypes N (neuraminidase) set the release of the virus that has been formed in the cells.

Whatever the type of flu, this illness can be dangerous and cause serious complications such as pneumonia. This risk of complications particularly affects older people and people suffering from other illnesses which put them at increased risk of pneumonia. In addition, flu is a serious public health problem.