Excellent discussion as always. I just wanted to add something about the disease that I'm honestly surprised Glenn missed. My immediate assumption was that the disease was supposed to be lead poisoning. Now, a quick Google search shows that the symptoms Caius had don't match lead poisoning. But I still think that's what Moore had in mind. For a long time there was an archaeological theory that lead poisoning caused the end of the Roman Empire - in other words, that the Romans used so much lead in their day-to-day lives that it eventually took its toll on the population. The theory still pops up from time to time, but it's largely discredited now. That discrediting only really got going in the 2000s, so the theory still would have been current when Moore was writing.
There are a couple of points that support my idea:
1) It was generally thought that Rome (as in the city itself) was the worst affected part of the empire, so Caius coming from the city and having symptoms while the citizens of Britain don't makes sense.
2) The disease reflecting the state of the empire: if one believed the theory that lead poisoning caused the end of the empire, this would literally be the case: the citizens are suffering from lead poisoning, and the empire is collapsing as a result.
Whether this is actually what Moore had in mind, I don't know. But I feel like it fits with the rest of the way he portrays the Roman Empire: not particularly historically accurate, but enough to pass muster for a non-expert.
Also, regardless of accuracy, I liked the bit about the Christians and Caius's idea to blame them he couldn't find the actual perpetrators. That to me at least fitted with the general attitude of persecution there still would have been towards Christians at the time, even if the circumstances the Christians are living in aren't right.