H1N1: is the emergence of the 2009 A (H1N1) virus an example of antigenic shift or antigenic drift?explain the difference between shift and drift.

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H1N1

1.Is the emergence of the 2009 A (H1N1) virus an example of antigenic shift or antigenic drift? Explain the difference between shift and drift.

2. Why was the 2009 A (H1N1) influenza epidemic considered a pandemic?