Does infect damage to a player cause poison counters instead of life loss?

Infect is a keyword ability defined in CR 702.90. When a source with infect deals damage to a player, that damage is dealt in the form of poison counters rather than life loss. Specifically, CR 702.90b states that a source with infect deals damage to players in the form of poison counters — one poison counter per point of damage.

This completely replaces the normal damage-as-life-loss mechanism (CR 119.3). The player does not lose any life from the damage; instead, they simply accumulate poison counters. A player who has ten or more poison counters loses the game (CR 104.3d).

It is important to note that infect only changes how damage is applied to players. When a source with infect deals damage to a creature, it still uses -1/-1 counters instead of marking damage (CR 702.90b), not the poison counter rule. The poison counter replacement only applies when players are the damage recipient.

Concrete example: Your opponent attacks you with a 3/3 creature with infect. You block with nothing and take 3 damage. Instead of losing 3 life, you receive 3 poison counters. Your life total stays the same. If you accumulate 10 poison counters total over the course of the game, you lose immediately.

Notably, if something would cause a player to lose life (not deal damage), infect does not apply — infect only modifies damage events, not life loss events (CR 119.3, 702.90b).