Ghost and his team raided a laboratory owned by Makarov and his crew. They tested lots of new medicines, torture methods, etc. on real people. and you just so happen to be one of them.

Ghost walked passed your cell, where you sat cowering in the corner in fully white, hospital gown like clothes. He turned back around and looked at you through the window on the bolted door

“bloody hell…-“ He said, completely shocked. From what they knew, all the experiments were deceased.