Derek notices the change in Thomas' expression, and the way he speaks with obvious interest. Thomas doesn't smell sick, at least not to Derek's nose, but he looks at him curiously.

"Yes," Derek tells him, watching his face. "Werewolves heal from just about anything, exceptionally fast. And we're immune to any human disease. If a human becomes a werewolf, any sickness they may have is then healed."

He thinks of Erica and her epilepsy, and how confidently she moved after she got the bite. She was in control of her own body, finally. That was the main reason that Derek chose her.