(Capcom's primary storyline)
Bats are an order of flying mammals. Three forms of bat have been encountered.
Infected Bats are any species of bat 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, fruit 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, a few bats in particular mutated to an enormous size. The mutations took a heavy toll on their body, causing multiple muscle tears in the animals' enormous wingspan.
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 optionally kill bats for gold. Bats in RE5 can rarely drop grenades. They also appear in Lost in Nightmares and Resident Evil: Revelations 2. Bats are seen swarming from a window in the demo version of Revelations.