Infect is a keyword ability that changes how a creature's damage is applied. Under CR 702.90b, damage dealt by a creature with infect to a player causes that player to get that many poison counters instead of losing that much life. The life total is completely unaffected.
A player who accumulates 10 or more poison counters loses the game immediately (CR 104.3c). This is tracked separately from life totals, so a player at 20 life with 10 poison counters still loses.
It's worth noting that infect affects damage to creatures differently: instead of marking damage, it puts that many -1/-1 counters on the creature (CR 702.90b). So infect replaces damage in two distinct ways depending on the target.
Concrete example: Your opponent attacks you with a 3/3 creature with infect. It deals 3 damage to you unblocked — you receive 3 poison counters and your life total stays exactly the same. If this happens three more times (totaling 10+ poison counters), you lose the game regardless of how much life you have.
Infect damage cannot be redirected to gain life either — effects like life-gain don't interact with poison counter placement, since no life loss is occurring in the first place.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.