This image depicts a Petri dish containing a MacConkey agar medium, which had been inoculated with Gram-negative Yersinia pestis bacteria. Y. pestis is the pathogen responsible for causing human plague. This was the appearance of the colonial growth after 24 hours of incubation.
People usually get plague from being bitten by a rodent flea that is carrying the plague bacterium or by handling an infected animal. Millions of people in Europe died from plague in the Middle Ages, when human homes and places of work were inhabited by flea-infested rats. Today, modern antibiotics are effective against plague, but if an infected person is not treated promptly, the disease is likely to cause illness or death.