As we enter a new year, many of us reflect on aging, particularly our appearance. We wonder, "Why do some of my friends look and function as if they are younger than me?"

Most academic research and popular writing on this topic focuses on genes, diet, stress, sleep, and exposure to alcohol, tobacco, and the sun. But a growing body of evidence points to another important and often overlooked factor: the number and type of infections you have had over your lifetime.

Every infection leaves a lasting change in the body. Viruses, bacteria, parasites, and other microbes stimulate our immune system to produce protective factors, such as antibodies. But these microbes also cause our immune system to release chemical signals that can damage tissues if they persist too long. Sometimes our immune system is unable to rid our body of a microbe. In that situation, the microbe can linger silently in cells or........

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