Ball Python Sick (HELP)

[Malum Argenteum]

Here's a link to a recent thread where someone's boa was diagnosed with IBD after 11 years, so it can happen.

Great link. Also, many snakes that carry nido are asymptomatic (reliable sources suggest as many as 10% of captive snakes of the relevant taxa are carriers). I don't know about the prevalence of asymptomatic carriers of ferlavirus (OPMV), and I'm not sure it is known.

Your vet should be able to screen for the three viral diseases you mention. That would be a good start in getting a diagnosis, but keep in mind that false negatives are possible (mostly from irregular viral shedding even when symptomatic).