A viral shift is the sudden emergence of a seemingly novel strain of a virus. It is a prerequisite viral condition for pandemics. An existing virus undergoes a change and becomes a pathogen, such as the constantly mutating "A" strain of influenza in which the change is known as an antigenic shift. Existing vaccines or natural antibodies cannot produce an immune response to the new strain.
Viral shift is caused by viral subunit reassortment, as opposed to simple random mutation.
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Viral shift Wikipedia(Text) CC BY-SA