Evolution is not a process toward better quality of life and life expectancy of individuals. As long as enough individuals can reach the age to procreate in their environment evolution is done. Evolution didn’t train our bodies to reject the diseases we already have the vaccines for neither, so your reasoning would apply to smallpox as well. And what about viruses appeared after Homo sapiens evolved (such as HIV)?

I don't think it works like that, from my recollection of the uni courses I did 20 years ago.

Even a small advantage like 1% will quickly propagate in a population, because it's about advantage over 1,000s of generations.

That this disease defence CAN be turned on, means some people would have at some point had a genetic mutation to turn it on.

As the GP pointed out, therefore it must be a net negative from an evolutionary stand point.

I also suspect it would be calorific consumption, as someone else said, so it might be ok.

However, there are plausible other explanations. For example there are medical conditions that result from a too aggressive immune system and it could instead be reducing the chance of that occuring.