As the first explorers pushed into the interior of the New World, they found native populations reeling from diseases never seen in the Americas. The bugs got there first. Leapfrogging ahead of the Europeans, the microbes carrying smallpox, measles and other diseases had decimated communities that had no natural immunity to them. Now, on a more distant new world, the bugs have done it again — across 50 million miles of interplanetary space. Decades will pass before the first humans set foot on Mars. But they won’t be the first Earthlings to land there.