Chuck was a poker player who participated in a high-stakes game which included Bernice Summerfield and was sponsored by the mob. He didn't have enough money to enter the game, so he sold some of his species' serum to the mob in exchange for a place in the game. The serum infected several humans, increasing their metabolism and causing them to attack and eat other humans.
When Chuck lost the game, he was to be killed, and the Seventh Doctor convinced him to cure the humans by letting them bite him, as the enzymes in his body counteracted the serum. (PROSE: Too Rich for My Blood)