Influenza is an infectious disease with mostly respiratory symptoms caused by influenza viruses. The most significant impacts of influenza viruses on humans are those arising from the influenza A strains. Seasonal influenza is a disease that annually affects Europe and the rest of the northern hemisphere during the winter season with larger or smaller epidemics. The southern hemisphere has a similar epidemic in its winter (June to October).

The disease can be anything from mild to very severe. Each year there are many avoidable deaths from influenza. Though death is considerably more common in older people and those with other illnesses (such as heart disease and chronic lung disease) severe disease and some deaths occur each year in healthy young and middle aged adults and children.

Because the viruses causing each year's epidemics are usually similar to the last year's it is possible to produce a vaccine for the coming influenza season with a good chance that it will be protective.