I guess my question is, if steroids can cause this much trouble with your immune system, could they cause something even worse like PML ?
Sure sounds like IV steroids and your system don't get along very well!!
PML is caused when the JCV virus (present in about 85% of us) is allowed to "grow" in our brains. Normally our immune system keeps the virus in check and it's not a problem.
There wasn't all that much discussed about PML until it was discovered in a few Tysabri users. Prior to that, it occurred often in AIDS patients who of course, have severely diminished immune systems. The connection between Tysabri and PML isn't known as yet other than Tysabri is an immune system altering drug that effects how the immune system operates in your brain. One theory is that MS patients, who have taken immunosuppressive or immune system altering drugs ( Steroids, Novantrone, CRABs etc) and then use Tysabri, get to a point that allows the JCV virus to "escape". Just what combination and what circumstance this can happen isn't known so far.
As to whether IV steroids, by themselves, can cause PML is likely a long shot. You've discovered that these steroids have caused you a severe digestive problem but I would think that you would have to compromise your immune system a lot more before something like PML could happen.
This, of course, is strictly my opinion and I don't have any medical training. I would think asking your doc the same question might be something you want to do as well.