The Black Death was an epidemic of bubonic plague, a disease caused by the bacterium Yersinia pestis that circulates among small rodents (like rats, mice, and squirrels) living in crowded colonies. People most commonly get plague after being bitten by a flea that is carrying the plague bacterium or by handling an animal infected with the disease. One can also become infected by inhaling respiratory droplets after close contact with people with pneumonic plague.

If left untreated, bubonic plague kills about 50 percent of those it infects. The other two forms are fatal without antibiotics. All three types are highly infectious, with bacteria disabling the body's defense cells by injecting them with toxins, but the pneumonic form is the most virulent of the three. If untreated, it has a very high fatality rate, and can kill within 24 hours.

While the bubonic plague that terrorized Europe and Asia in the 6th and 14th centuries is almost extinct today, a few countries—most of them in Africa and Asia—have reported cases in the last decade. Zambia, India, Malawi, Algeria, China, and Peru are among those affected by the disease since 2001, with the Democratic Republic of the Congo topping the list with more than 1,100 cases, according to the World Health Organization (WHO). Between 1,000 and 2,000 cases each year are reported to the WHO, though the true number is likely much higher. It is hard to assess the mortality rate of plague in developing countries, as few cases are reliably diagnosed and reported to health authorities. WHO cites mortality rates of eight to 10 percent, but WHO studies suggest that they may be much higher in some plague endemic areas.

The number of cases of plague reported in the United States is small, with only one case in 2003 and two each in 2001 and 2002, none of which were fatal. The decline followed an increase in plague rates in the 1990’s, with a high of 14 cases reported in the U.S. in 1994. In recent decades, an average of seven human plague cases have been reported each year, most of them in the bubonic form, according to the Centers for Disease Control and Prevention.

As recently as December 2013, the island of Madagascar battled a new plague outbreak, which reportedly killed 32 people, out of 84 suspected cases. Sixty of the cases were thought to be pneumonic plague. Madagascar reported 60 deaths from bubonic plague in 2012, attributing the spread of the disease to poor hygiene and declining living standards due to an ongoing political crisis in the country.

As 2013 drew to an end, Madagascar officials and charities faced one major concern: that the deadly disease could spread through the island's prisons, which are infested with rats. Officials warned that rats living in the prisons, carrying fleas that can transmit the disease to humans, could cause the outbreak to expand beyond the prisons' walls.