What is the term given to an overly aggressive immune system response?

Question

The spanish flu in 1918 affected and killed mostly young, healthy people. The theory is that the body's immune system response caused their deaths. What is the name of this term?

Answer 1

Someone who has an overactive immune system -- one that does not just attack diseases but also their body-- has Autoimmune disease.

Answer 2

I'd say it's called an allergic reaction.