Even in areas of the world that still experience outbreaks, mortality rates from plague are estimated to be eight to 10 percent. The disease that caused the most lethal epidemics recorded in history and killed 60 percent of the people in medieval London and Florence is no longer on the most feared-killers list.
"O happy posterity, who will not experience such abysmal woe and will look upon our testimony as a fable," wrote Renaissance poet Petrarch, who lost his beloved and muse, Laura, to the "Black Death." Petrarch, a native of the Republic of Florence, witnessed a terror we can only find in end-of-the-world Hollywood blockbusters today: The lucky few who had escaped the Black Death spent most of their time carrying dead bodies and burying them by the thousands in pits around churches and elsewhere in the cities. The corpses lay on top of each other, separated by thin layers of clay, "just as one makes lasagna with layers of pasta and cheese," in the words of a 14th-century Florentine chronicler whom historian Ole J. Benedictow cites in his studies on the impact of Black Death.
Researchers have identified three types of plague, characterized by different symptoms and means of transmission. The bubonic plague, which is assumed to have been the chief killer in medieval outbreaks, causes painful, swollen lymph nodes (called buboes) around the groin, armpit, or neck. Septicemic plague spreads in the blood stream and comes from flea bites or contact with plague-infected body tissues. The pneumonic (or pulmonary) plague is an advanced stage of bubonic plague when the disease is passed directly, person to person, through airborne droplets coughed from the lungs.
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.
The medieval pandemic that swept through Asia and Europe in the late 1340s, which historians estimate killed anywhere from 25 million to 100 million people, is also understood to have spread via rats and fleas. But new evidence from skulls in central London suggests plague must have gone airborne to spread that quickly.
British forensic scientists and archaeologists examined 25 skeletons of plague victims unearthed in Charterhouse Square in central London a year ago. The skeletons were recovered from a mass burial site lost for hundreds of years, where a large number of them were neatly buried in layers. The team compared human DNA from teeth with samples from the recent outbreaks in Madagascar. The samples were an almost perfect match, suggesting that the medieval plague was no more virulent than the contemporary strain.