Corpse Cur has an enters the battlefield triggered ability that allows you to target a creature card with Infect in your graveyard and return it to your hand. Viridian Betrayers has a static ability that can give the Betrayers itself Infect. However the Betrayer’s static ability only works then the Betrayers are in play. The Betrayers will NOT be be given infect when in any other zone. This means that you cannot target a Viridian Betrayers with Corpse Cur’s ability even if an opponent has a poison counter.