Quick-Play Virus Cards are a type of Virus Card, and can be activated during any phase of your turn as well as during your opponent's turn (provided it was Set beforehand), similar to Spell Cards. Cards like "Dragon Virus" are examples as Quick-Play Spell Cards.
Quick-Play Spell Cards may be activated from your hand during your turn; they cannot be activated from your hand during your opponent's turn. You may also chain Quick-Play Spell Cards to cards that your opponent activates during any phase of your turn and, provided that they are face-down on the field, during your opponent's turn. Quick-Play Virus Cards cannot be activated on the same turn they are Set (This goes for ones Set during your turn, as well as during your opponent's turn, such as by the effect of Dust Tornado).