Calvin is a healthy kid aside from his talent for getting into trouble. He's visited the doctor and is mortally terrified of shots, been home from school with the flu, gotten stung by bees and scraped his knees. Normal stuff for his canon. Mentally, there has been speculation that his short attention span and hyperactivity might be the result of some kind of condition, but I don't feel that's the case.

In-game, he's been exposed to more of the City's dangers than he ever would have wanted. He got smacked by Godzilla once (long story) and experienced a terrifying encounter with the Other Mother from Coraline that he still has nightmares about. Other than that, he remains largely unscathed.