In the Northern Hemisphere, seasonal influenza epidemics usually occur during the winter ("the flu season") with the peak typically around January and February. At-risk groups should be vaccinated annually well in advance (October-November) before the start of the expected influenza epidemic. National public health organizations issue recommendations of groups to whom annual influenza vaccination should be offered (possibly free of charge), i.e. of individuals whose health would be essentially threatened by influenza or who would gain significant health benefit from influenza vaccination. Influenza can be diagnosed on the basis of the clinical presentation combined with a confirmed epidemic circulating in the community, or on the basis of virological (rapid) diagnostic test results. The treatment of influenza in a healthy adult patient is generally symptomatic. Antiviral agents should be considered for children, pregnant women, patients with exceptionally severe symptoms and those belonging to at-risk groups. The sooner the drug therapy is started, the better the effect. Treatment should be started within 48 hours of symptom onset.