In Cecil’s first tome (p. 216 in my copy), he tackles an educated guess as to the identity of “Herod’s Evil”, a disgusting-sounding disease. CA reports the likely diagnoses as [severe, advanced] arteriosclerosis or diabetes.
One other possibility ? One of the filovirus family, such as Marburg. There was a great deal of traffic in African wildlife to the Roman court, and the least virulent of the strains would seem to cause those symptoms… Marburg didn’t always turn active immediately, nor is it always fatal - see “The Hot Zone”, and there would be some amusing symmetry to a Roman emperor becoming sick after playing with his primate.
“Proverbs for Paranoids, 1: You may never get to touch the Master, but you can tickle his creatures.”
- T.Pynchon, Gravity’s Rainbow