Thomas was an elderly worker employed by the Umbrella Corporation; assigned to work in the scrap yard behind Raccoon City's hazardous waste disposal center. Thomas, as a person, was described as being caring and kind to his fellow employees. Thomas was also a professional chess player; loving the game so much he fashioned his keys into chess pieces, and took to playing matches against his co-workers.
Thomas was infected with the Tyrant virus during the Arklay Mountains Outbreak when it reached Raccoon, and gradually got worse as time went on. In order to stop himself from thinking about consuming human flesh, Thomas constantly smelt and devoured his roast beef sandwiches, which he brought with him to work, and his personality remained active for longer than usual - suggesting he was infected through water.
Inevitably, Thomas succumbed to the virus and became a Zombie.