This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
Influenza pandemic is due to 
Select and explain. 
A. Antigenic shift, B. Antigenic drift, C. Different strains, D. All
A: Antigenic shift
Answer: A: Antigenic shift 
Explanation: Ans. a (Antigenic shift). (Ref. Anantanarayan, Microbiology, 4th/pg.487)TermCommentsAntigenic drift- Gradual sequential change at frequent interval i.e. mutation of pathogen (e.g. Influenza A), such that the surface antigens differ from those of previously existing strains.- Accounts for small epidemics.Antigenic shift- Sudden drastic discontinuous change in antigenic structure i.e., by recombinant DNMA technology. - Accounts for major epidemic and pandemics.