When a chance mutation modifies a virus to attach to a new host species, it is called

gain-of-function

Explanation:

Gain-of-function mutations represent a type of genetic change that may confer a new function to the gene that undergoes the mutation. In this case, a gain-of-function mutation conferred to the virus an adaptive advantage by allowing it to encode a new protein which is capable of attaching to the cell membrane of a new host. In this regard, it is important to note that loss-of-function mutations are quite common in nature, while gain-of-function mutations are relatively rare.

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