This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
Influenza virus causes new epidemic by (3-5 yrs)- 
Select and explain. 
A. Antigenic drift, B. Antigenic shift, C. Cycle trends, D. Mosaicsm
A: Antigenic drift
Answer: A: Antigenic drift 
Explanation: 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