In 1796, English physician Edward Jenner infected a young boy with cowpox. Later, when he injected the child with the deadly smallpox virus, he did not get sick. And thus, the first vaccine was born, saving millions of lives and immortalizing cows in public health. (The word vaccine is derived from the Latin word vacca for “cow.”) Or so the legend goes. But the story is probably wrong, according to a report published today in The New England Journal of Medicine. That’s because the vaccine used to prevent smallpox was likely horsepox, not cowpox, researchers say. The latest bit of evidence comes from the historic containers above, which held a smallpox vaccine manufactured by the Philadelphia, Pennsylvania, company H. K. Mulford in 1902. Although the vaccine was manufactured more than a century after Jenner’s breakthrough, it’s the oldest vaccine strain analyzed so far. When scientists sequenced its DNA, they found it most closely resembled the genome of horsepox. Jenner himself wrote that he used material from both cows and horses in his experiments, and the new finding suggests that it may be horses, not cows, we have to thank for the world’s first vaccine. Or perhaps we should say equusine?