A mod for Madness Interactive reloaded which allows you to become a zed and infect other characters in order to turn them into zeds like you.
Download it from the green 'Code <>' button at the very top. The mod is installed like any other mod, extract it and pop it into your Mods folder.
If you have 0.50.0 installed, the mod should also work on that version, it should also probably have the newest animations.