Mutation is seen in
A unique feature of the influenza virus is its ability to undergo angenic variaons.
Angenic variability is highest in influenza virus Type A and less in Type B, while it has not been demonstrated in type C.
Surface angens hemagglunin and neurominidase, undergo independent angenic variaons which may be of Two types.

1. Angenic dri

Angenic change is gradual over a period of me
The new angens, though different from the previous angens, are yet related to them,
Angenic dri involves point mutaon in the gene owing to selecon pressure by immunity in the host populaon.
Angenic dri accounts for the periodic epidemics of influenza.
Occurs every 2-3 years in case of Influenza A and every 4-7 years in case of Influenza-B 2.

2. Angenic shi

An abrupt, drasc, disconnuous variaon in the angenic structure,
Result from genec recombinaon of human with animal or avian virus,
Accounts for the major epidemics or pandemics,
Occurs every 10-15 years,
Occurs in influenza ‘A’ only
Poliovirus
Influenza virus
Mumps virus
Measles virus