Something along the lines of this happened to me today:
Turn 1: Simic guildgate
Turn 2: swamp, Blighted Agent
Turn 3: Island, Skeletal Grimace. Attack for 2 unblockable infect
Turn 4: Forest, Ana Sanctuary, Attack for 2 infect again
Turn 5: Blighted Agent is now a 7/7 for this turn, due to having a blue and black permanent. Opponent was suddenly on 11 poison counters.