"I have never really been ill"

Philo of Alexandria
Public DomainPhilo of Alexandria - Credit: André Thevet
Claudius suggests that Caligula endured a period of physical illness, from which he emerged insane. His main source for this seems to be Philo of Alexandria's Embassy to Gaius.

Philo visited Caligula in Rome as part of a deputation of Jews, protesting the Emperor's insistence that he be worshipped as a god (a bigger problem for the monotheistic Jews than it was for adherents of most polytheistic religions around the Empire). In Graves's fictionalised account, Caligula wakes from his long illness convinced of his own divinity.