(Capcom's primary storyline)
Rats living in the sewers of Raccoon City were the original carriers of the t-Virus that caused the Raccoon City Destruction Incident - as opposed to the leeches, which caused the outbreak in the Arklay Mountains. The only known example of an attack on a human was on an injured U.S.S. member seen in the introductory movie to Resident Evil Outbreak.
According to the BIOHAZARD 2 Official Guide Book, the rats in the sewers grew significantly from the t-Virus, but were killed-off by their enemies, the Large Roaches.
Non-infected rats appear in various locations in Resident Evil 5 and can be killed by the player. They also appear in Resident Evil 6 and also can be killed by the player.
Infected Rats in Raccoon City.
Infected rat in Resident Evil Outbreak.