[Rexxar]

Maybe it would made the immune system age faster if it is "used" too much.

[ekianjo]

Inflammation is certainly not "free". It causes systemic damage.

[tensor]

So does getting infected over and over. Much worse damage. Evolution isn't some magic thing that gives you the most optimal creature for a given metric. The only metric is procreation. Not longevity. Not a pleasant life.