in the 14th century, the bubonic plague killed more than 2/3 of europeans. the plague was transmitted by fleas who were infected by rats and would inject a bacteria into the bloodstream that shuts down a person's immune system. while not as common today, the bubonic plague still exists, infecting around a thousand people a year. there are about seven cases in the united states each year, many of those in arizona and new mexico. scientists have discovered that the bacteria works by affecting signaling pathways in the immune system.