Many diseases are also caused by an over-active immune system in one way or another, like Rheumatoid arthritis, Celiac disease, and many others. It's an entire category of diseases.

https://en.wikipedia.org/wiki/List_of_autoimmune_diseases

If this works by cranking your immune system up to 11 and that's it, this won't be good for most people would be my bet. Worst of all, some issues wouldn't show up until years later. Many autoimmune diseases aren't exactly as obvious as the flu.