Does infect damage to players use poison counters and damage to creatures use -1/-1 counters at the same time?

Infect is a keyword ability defined in CR 702.90. It changes how a source with infect deals damage in two distinct ways depending on the target: damage to players is dealt as poison counters instead of loss of life, and damage to creatures is dealt as -1/-1 counters instead of normal damage marked on the creature. Both replacements apply at the same time based on what is being damaged.

This means if an infect creature attacks and is blocked, it deals -1/-1 counters to the blocking creature (CR 702.90b), while if it were dealing damage directly to a player, that player would receive poison counters instead (CR 702.90a). A player with 10 or more poison counters loses the game (CR 104.3c).

Importantly, -1/-1 counters from infect are permanent reductions to the creature's power and toughness — they do not go away at end of turn like regular damage does (which is removed during cleanup, CR 514.2). This makes infect significantly more dangerous against creatures than regular damage.

Concrete example: You control a 2/2 creature with infect and attack your opponent who blocks with their own 3/3. The infect creature deals 2 damage to the blocking creature as two -1/-1 counters, making it a 1/1 permanently. Simultaneously, if the 3/3 had trample and 1 damage hit the player, that player would get 1 poison counter instead.

Note that infect does not change how other effects interact with the source — it only replaces the way damage is marked or applied (CR 702.90b). Non-damage effects like deathtouch still function normally alongside infect.