My personal experience is your immune system is sharp on both sides. You hope whatever antibodies and immune response gets triggered doesn't cross react with self.

I have some friends that had Hashimoto's disease. Type 1 diabetes. Friends with lupus. 4-5 friends that had Guillain-Barre syndrome. My mom died of ITP. I have sarcoidosis. I have friends who for unexplained reasons are 'unwell'

ITP is an interesting one. The antibodies target an enzyme that's needed to keep your blood platelets from binding together in your blood. Thus the idea that you can have autoimmune syndromes that can mess with thousands of different enzymes and proteins. Or stimulate inappropriately. Seriously why not.

I can totally believe that infection with childhood diseases of yore could lead to dementia later in life.