Does a creature dealt damage by a source with infect get -1/-1 counters or regular damage?

Infect is a keyword ability defined in CR 702.90. When a source with infect deals damage to a creature, that damage is not marked as regular damage. Instead, the creature receives that many -1/-1 counters. This is a replacement effect — the damage event is replaced entirely by the placement of counters.

This matters significantly for game mechanics. Regular damage on a creature is removed at the end of turn cleanup step (CR 514.2), but -1/-1 counters remain permanently until something removes them. A creature that survives infect damage will be persistently weakened, potentially dying later if its toughness is reduced to zero by accumulated counters.

Note that infect works differently depending on what is being damaged. Against creatures: -1/-1 counters. Against players or planeswalkers: poison counters instead of regular damage (CR 702.90b). So the replacement effect has two different outcomes depending on the target.

Concrete example: You attack with a 2/2 creature with infect. Your opponent blocks with a 3/3 creature. After combat damage, the 3/3 does NOT take 2 damage — instead it gets two -1/-1 counters, becoming a 1/1 permanently. Your 2/2 takes 3 normal damage and dies at end of turn (since it has no infect itself).

Crucially, the infect creature's own damage to other creatures is replaced by counters, but damage dealt to the infect creature from sources without infect is still regular damage (CR 702.90, 119.3).