Influenza is an infectious disease caused by a virus. It is possible to have influenza more than once.
Which statements explain why it is possible to have influenza more than once?
1- The viral antigens change as a result of mutations.
2- The immune system may be weak and make few B-memory cells.
3- Untreated HIV infection has resulted in a low T-helper cell count.
1, 2 and 3
2 )
1 and 2 only
3 )
1 and 3 only
4 )
2 and 3 only
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