Influenza epidemics occur virtually every year, although the extent and severity of such epidemics vary widely. Sometimes pandemics occur (see below). The most frequent, extensive and severe outbreaks are caused by the influenza A virus. Illness and death from influenza outbreaks continue to be substantial. In the interpandemic period, death occurs primarily in individuals with underlying diseases who have been characterized as being at "high risk" for complications of influenza. It is estimated that approximately 10-15% of people get influenza around the world each year. During major epidemics, the attack rate of influenza may be as high as 50%.