It can be transmitted by blood, often through flea bites. The black rat, which was very common in the Mediterranean basin in the Middle Ages, was often be infected but the infection wouldn’t hurt the rat. However, fleas living on the rat could then bite humans, and thus transmit the decease. In humans the decease has a 30-50% mortality rate.
The infection can also arrive through the lungs, as pneumonic plague. In this case the mortality rate is higher still.
An infection of the black plague initially resembles a very bad flu, with high fever and muscle pains. When the infection reaches the lymph nodes in the groin and armpits, they swell and go black. This is what has give the yersinia pestis infection the names black plague or bubonic plague. The swelled lymph nodes can burst open releasing a pus containing the bacteria.
Often victims get gangrene in the fingers, toes, lips and nose, which will turn blue or black.
Epidemics of the black plague hit Europe from the 6th century until the 17th century, when it disappeared.
The people of Europe had little understanding of the medical and scientific reasons for the plague, but tried to prevent the spread of the decease through surveillance and quarantine.
The black plague still exists outside of Europe. There have been recent cases in North America, Central Asia and Madagascar. It can be treated with antibiotics.
In Europe the brown rat, which doesn’t carry the bubonic plague, has mostly supplanted the black rat. This is one of the reasons why the bubonic plague subsided in Europe during the 18th century.
Read more about the Lazzaretti Veneziani – the plague hospital and quarantine station in the Venetian lagoon.