Infect is an ability released in Scars of Mirrodin block. This ability mixes two past abilities together, reviving one and bring another back from a more or less recent set. Infect works in two ways. When a creature with Infect deals damage to an opponent, they get Poisonous Counters. When an opponent has 10 Poison Counters, they lose the game. When dealing damage to creatures it deals it in Wither damage or in -1/-1 counters.
If a creature has infect it deals no "damage" when it hits an opponent. If a creature you control deals 2 damage to an opponent it gives them 2 Poison Counters and their life stays the same.