Infect is a new keyword that will be introduced in Scars of Mirrodin. It was spoiled on 8/19/2010 via an easter egg in Wizards' Arcana section. When activated the easter egg displayed the following text:

This keyword ability signifies that the creature it is on, or it is bestowed on, deals damage to creatures in the form of -1/-1 counters, and damage to players in the form of poison counters rather than through loss of life.