In some of the Resident Evil games, bats are among the various animal species that have been accidentally infected by the T-virus. As irregular mutations go, these creatures are on the lower end of the spectrum. The T-virus has caused only a minute increase in the bats' body mass, though like all infected creatures, their aggression has been significantly heightened. They will attack in large groups and attempt to drain the blood of their prey. It is highly probable that their bites also pass on the T-virus to their victims.
Resident Evil Zero
During the Ecliptic Express Incident, Rebecca Chambers and Billy Coen were attacked by swarms of infected bats in a chapel on the grounds of the Umbrella Research Center. On their own they posed little threat, but a much larger specimen was also present and seemed to be capable of spawning more of its smaller brethren.
Resident Evil - CODE: Veronica
Claire Redfield and Steve Burnside encountered a number of infected bats within Alfred Ashford's residence on Rockfort Island. Claire managed to avoid fighting the creatures as she was able to keep them at bay with a lighter. The light from the lighter's flame seemed to ward off the bats and they would not approach.
Resident Evil: The Darkside Chronicles
Leon Kennedy and Jack Krauser encountered several infected fruit bats during Operation: Javier. These creatures supposedly became infected after feeding on fruit plants that had been contaminated with the T-virus.