I knew Eero Mäntyranta had magic blood, but I hadn’t expected to see it in his face. I had tracked him down above the Arctic Circle in Finland where he was—what else?—a reindeer farmer.

He was all red. Not just the crimson sweater with knitted reindeer crossing his belly, but his actual skin. It was cardinal dappled with violet, his nose a bulbous purple plum. In the pictures I’d seen of him in Sports Illustrated in the 1960s—when he’d won three Olympic gold medals in cross-country skiing—he was still white. But now, as an older man, his special blood had turned him red.

Mäntyranta, who passed away in late 2013, had a rare gene mutation that spurred his bone marrow to wildly overproduce red blood cells. Red cells convey oxygen to the muscles and the more you have, the better your endurance. That’s why some endurance athletes—most prominently Lance Armstrong—inject erythropoietin (EPO), the hormone that cues your bone marrow to produce red blood cells. Mäntyranta had about 50 percent more red blood cells than a normal man. If Armstrong had as many red blood cells as Mäntyranta, cycling rules would have barred him from even starting a race, unless he could prove it was a natural condition.