Bats are an order of flying mammals. Three forms of bat have been encountered.
Infected Bats are Vampire Bats infected with the t-Virus. They were first seen in the Umbrella Executive Training Center, where they had taken up residence in the church above the laboratory following the outbreak caused by James Marcus. The virus caused the bats to exhibit much more aggressive behaviour, as well as a slight increase in body mass.
There were also infected bats on Rockfort Island during the viral outbreak, but these showed no sign of increased size and could be deterred by light or usually avoided by walking instead of running around them. Simply equipping the Lighter will prevent the bats from harming the player.
During the 2002 outbreak in South America, bats which fed on t-Virus-infected fruit plants also became infected, displaying the standard heightened aggression and a small increase in size.
Although typically the only sign of infection was increased aggression, one bat in particular mutated to an enormous size. The mutation took a heavy toll on its body, causing multiple muscle tears in its enormous wingspan. This infected bat also had the ability to spawn multiple offspring at an alarming rate. The creature was fought and destroyed by Rebecca Chambers and Billy Coen.
Bats were also seen by Leon S. Kennedy in his exploration of the village and the nearby castle. In Resident Evil 5, Chris and Sheva can kill optional bats for gold. Bats in RE5 can drop grenades which is rare. They also appear in Lost in Nightmares. Bats are seen swarming from a window in the demo version of Revelations.