As someone with Lupus, you're misunderstanding these things entirely. Have you considered becoming the head of the HHS is the US?

Also this is a question, so it's not dumb.

Autoimmune disorders cause the immune system to not recognize anything. They can't tell friend from foe, and default to attacking everything.

Measles causing immune system amnesia by attacking and killing where the immune system stores memories.

The immune system still attacks everything, it just doesn't remember what works on every virus.

So it will still attack friendly parts of your body, because it's "targeting system" still just tells it to blast everything that moves.

And because it doesn't remember how to fight actual threats, it generated a larger and less effective immune response, then all those immune cells attack random stuff, which leads to measles making any autoimmune issue even worse.