Plague does play a role in the books. The Great Spring Sickness is mentioned, a huge and terrible plague that swept across the Seven Kingdoms about 90 years before A Game of Thrones and killed hundreds of thousands of people. Something like a quarter of the population of King's Landing died (including King Daeron II) and half the city was torched before it was stamped out.
Also (middling ADWD spoiler):
But yeah, plague as it is now is probably a bit overpowered in the game. If it can be toned down, that would be great.