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. Cyclic trends, D. Mosaicsm
B: Antigenic shift
Answer: B: Antigenic shift 
Explanation: Antigenic variations in Influenza Antigenic shift Antigenic drift Occurs due to Genetic recombination/reassoment / Rearrangement Point mutation Nature Sudden Gradual / insidious May lead to Epidemics/ pandemics Sporadic cases Ref: Park 25th edition Pgno : 166-172