A Closer Look at Who’s to Blame for the Black Death

In the 14th century, the Black Death ravaged Europe, killing over a third of the continent’s population. This pandemic was caused by the bubonic plague, a bacterial infection characterized by swollen, painful lymph nodes called buboes. 

The plague is caused by transmission of the bacterium yersinia pestis. According to genetic research, this microbe evolved in China over 2,600 years ago and has “followed humans around the globe.” Untreated this bacterium will kill between 50-90% of those infected, but with treatment, mortality ads drop to about 15%.

The methods of transmission of this bacterium are likely the main cause of its prevalence in the 14th century. These microbes travel on fleas that pick up the bacterium from infected rats, as rats are highly susceptible to plague, just as humans are. These fleas then spread the disease when they attempt to feed off another host. Thus explaining why outbreaks among humans are often accompanied by rodent deaths, known as rat falls.

The top photographs display a petri-dish culture of the bacterium and a microscopic image of the bacteria in blood cells, while the bottom two photos show the black rats and fleas known for spreading these microbes to humans.