Does this make you immune to your own crimson scourge? The helpfile makes me believe it does. I wasn't sure if there is some other affect/disease with power word decay that maybe this is talking about?

From what I recall you never catch your own scourge EXCEPT that once someone who was scourged is dead, I don't think the game knows it is your scourge any more.

So you can still catch it when you loot a corpse.

Last necro I played was a drow, and I took cryptborn. That pretty much did the trick and I almost never caught any disease or my own scourge again.