C-23 was a virus that attacked those with weak immune systems. It killed many children, a few middle aged and elderly people in Forthaven.
Richard Tate lost both his sons to the virus, which led to his wife's suicide. At the time of the out-break, the ACs were thought to be the source of the virus, and Tate ordered them to be expelled from the colony.
It was later revealed that Tate also ordered the execution of the Acs, but Mitchell Hoban disobeyed that order and let them go free.