What, in his life, cause Harold Ramis to get an infection in May 2010?

It wasn't really the infection that killed him, it was the disease. I myself am diagnosed with the same disease.

The infection could have been viral, but it happens to trigger the immune system into attacking the body. The blood vessels become inflamed and can attack vital organs. I experienced something similar when a bronchial infection caused my ANCA protein antibodies to multiply and attack my kidneys.

Unfortunately for Harold, it's very hard to bounce back from a triggered influx when you're approaching 70.

~ I'm a 21st century man and I don't wanna be here.

To my understanding, such infection is natural, as are all other autoimmune diseases.

Not sure what you mean by natural. What I was trying to say was that some doctors theorize that a viral infection can trigger a flare up of the autoimmune disease (in Mr Ramis's case, Vasulitis). In my case, I was diagnosed with pneumonia before they found out I had Vasculitis. So they weren't sure if I caught some lung infection that triggered it, or if the disease (or the protein/antibodies) just flared up by themselves with no "trigger." No one knows why it occurs.

~ I'm a 21st century man and I don't wanna be here.

I see. Well, thanks for the explanation.

I'm not a doctor though, so don't take it as some experienced medical advice ;)