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
[{'content': 'Influenza virus causes new epidemic by (3-5 yrs)- \nSelect and explain. \nA. Antigenic drift, B. Antigenic shift, C. Cycle trends, D. Mosaicsm', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]