Aimed at the self, 'Useless' Infestation easily let one circumvent the 6-copy clause creatures have and/or buy some time from the creeping deckout. Aimed at an enemy without source of Life quanta, UV/UI will screw it.
The extra five Relics/Giant Frogs are placed into the target player's deck, regardless if the deck contained 0, 5, 15, or 50 cards, am I correct?