(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.
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 Resident Evil: Revelations 2 and can also be killed by the player, in the latter case, killing rats is one of the requirements for a medal in Episode 4.