During Infections, if a Mutation card is drawn (and the purple disease is not eradicated), an Infection card is drawn from the bottom of the Infection Deck and 1 purple cube is placed on that city. Then both cards are discarded to the Infection Discard Pile
If the purple disease is eradicated, nothing happens. The card is simply discarded to the Infection Discard Pile. It still counts as 1 Infection card draw for the turn.