Influenza virus causes new epidemic by (3-5 yrs)-

The influenza A virus is unique among the viruses because it is frequently subject to antigenic variation, both major and minor . When there is a sudden complete or major change , it is called a shift, and when the antigenic change is gradual over a period of time , it is called a drift . Antigenic shift results from genetic recombination . Antigenic drift involves point mutation . Ref:PARK&;S TEXTBOOK OF PREVENTIVE AND SOCIAL MEDICINE , 23rd edition , Page :154