In 1545 and 1576, people in the Mexican highlands started dying. They had red spots on their skin. They vomited and bled. The death toll was huge. Between 7 million and 17 million people died. For a long time, some scientists thought they all had smallpox or plague. Others thought it might be typhoid fever. But there was no way to be sure, because infectious diseases don’t leave evidence. A new study might point to the answer. Researchers analyzed DNA from the teeth of people who died in the epidemic. They found bits of ancient salmonella DNA.