How does infect work in magic the gathering?

Answer 1

Infect is a replacement effect, meaning it modifies how the normal game rules work.

Infect specifically modifies the damage-dealing rules. A creature with Infect does not deal regular damage to creatures or players. In the case of a creature, it places a -1/-1 counter on it for each point of unprevented damage. In the case of a player, it places that many Poison Counters. This is for combat damage and effect damage too.

If a creature with Infect deals damage to a Planeswalker, as a Planeswalker is neither a creature nor a player, then the damage is handled normally. The Planewalker's own replacement effect will removes some of his or her loyalty counters.