Infect changes how a creature deals damage: creatures receive that many -1/-1 counters, while players receive poison counters instead of losing life. A player with ten or more poison counters loses the game.
Inkmoth Nexus provides an evasive infect threat from the mana base, while pump spells such as Might of Old Krosa and Berserk can turn a tiny infect creature into a sudden kill. Proliferate offers another way to increase poison after the first counters land.