Our biological systems aren’t perfect, not by a long shot. They respond well in certain situations, but they can’t possibly react appropriately in all situations. Evolution is far too slow for these systems to keep up with every change.
There will be circumstances where an immune reaction is not desirable and causes damage. The only thing that matters to evolution is that enough individuals of a species survive to propagate the species. A few dying in extreme situations won’t necessarily prompt adaptations.
A lot of the time, it’s about the lesser of two evils. It’s better to risk complications due to a fever that’s too high, than not respond aggressively to a virus.
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