<prompt> Influenza pandemic is due toAntigenic shiftAntigenic driftDifferent strainsAll  <response>: Ans. a Antigenic shift. Ref. Anantanarayan, Microbiology, 4thpg.487TermCommentsAntigenic 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. 