Most people can name their heart, lungs, and kidneys without hesitation. Almost nobody thinks about the thymus. It sits above the heart, does its most visible work before puberty, and then spends the rest of a person's life shrinking while medicine assumes it has retired. Two studies published in Nature by Mass General Brigham researchers just analyzed CT scans from more than 25,000 adults using AI and found that the health of this overlooked organ predicts longevity more powerfully than most tests doctors currently run. People with the highest thymic health scores had a 50% lower risk of dying from any cause, a 63% lower risk of cardiovascular death, and a 36% lower risk of developing lung cancer. A separate analysis of 3,400 cancer patients found that thymic health predicted immunotherapy success better than tumor type or age. The organ medicine wrote off after childhood has been quietly determining whether adults live or die for their entire lives.