An interesting paper: "Recent studies suggest that activation of the peripheral immune system elicits a discordant central (i.e., in the brain) inflammatory response in aged but otherwise healthy subjects compared with younger cohorts. A fundamental difference in the reactive state of microglial cells in the aged brain has been suggested as the basis for this discordant inflammatory response. Thus, the aging process appears to serve as a 'priming' stimulus for microglia, and upon secondary stimulation with a triggering stimulus (i.e., peripheral signals communicating infection), these primed microglia release excessive quantities of proinflammatory cytokines. ... there is a propensity for this response to be maladaptive in aged subjects, resulting in greater severity and duration of the sickness behavior syndrome." Your immune system evolved for a life span of a few decades, optimized to help you live long enough to pass on genes. It's all downhill after that, as some of those optimizations start to be actively harmful later on in life. Repairing these deficiencies in the aging immune system is an important component for future longevity science.