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Instead of relying on a steady stream of new cells, the body must depend more heavily on the T-cells it already has. These are the veterans—cells that have encountered specific pathogens before and "remember" how to fight them. Over decades, however, these memory cells can begin to show signs of a phenomenon known as cellular exhaustion. They remain present in the blood, but they lose their vigor, responding more sluggishly to the very threats they were once specialized to destroy. This exhaustion is one reason why the body occasionally forgets its past immunities. The molecular library of defensive strategies, once sharp and reactive, begins to blur. Even if a person was exposed to a specific illness years ago, the aging immune system might not mount a defense fast enough to prevent a second infection. The precision of the adaptive immune system—the part of our biology that learns and evolves—gradually begins to fade. As the adaptive system recedes, the body begins to lean more heavily on its innate immune system. This is the older, more primitive layer of defense that doesn't learn from experience but instead launches a broad, non-specific attack against any perceived threat. While this system remains active, its lack of precision can create its own set of problems. It is often less effective at fully clearing a specific pathogen and can sometimes cause collateral damage to healthy tissues. The World Health Organization notes that these baseline changes in immune function contribute to a state of chronic, low-grade inflammation throughout the body. This persistent "background noise" of the immune system can interfere with the repair of other organs, creating a feedback loop where the very system meant to protect the body begins to weigh it down. It is a transition from a targeted, surgical defense to a more generalized and tiring state of constant alert.