Researchers presented findings last week on two scourges that left millions of victims in their wakes—the Black Plague and the series of epidemics which wiped out native Mexicans in the 16th Century, known as cocoliztli. The Black Plague killed an estimated 25 million Europeans and was thought to have been mainly spread by rats. But the new research applied current understandings of disease transmission and found a human model of transmission fit a lot better. Over in the New World, the cocoliztli outbreaks killed over 15 million, but researchers hadn’t pinned down the exact cause. Last week’s study puts forth Salmonella enterica, a bacterium, as a possible main offender. Good thing these contagions are in the past right? Except they’re both still present in the modern world. That’s cool.