<prompt> True regarding antigenic shift -GradualSeen in influenza A BDue to genetic recombinationDue to point mutation  <response>: INFLUENZA:- Influenza viruses classified under the family ohomyxoviridae. 3 viral subtypes-A,B and C. These three are antigenically distinct.No cross immunity between them. Influenza A and B - responsible for epidemics throughout the world. Influenza A has 2 distinct surface antigens- hemagglutininH and neuraminidaseN. The influenza A virus is unique among the viruses because it is frequently subjected to antigenic variation, both major and minor. ANTIGENIC SHIFT:- 1. Sudden complete or major change. 2. Results from genetic recombination of human with animalan viruses providing a major antigenic change. 3. Can cause major epidemicpandemic involving most all age groups. ANTIGENIC DRIFT:- 1. Antigenic change is gradual over a period of time. 2. Occurs due to point mutation in the gene owing to selection pressure by immunity in the host population. 3. Affected by previous antibodies. Reference: park;s textbook of preventive and social medicine, 23rd edition, pg no.154 