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.
The DNA samples were subjected to a brand new scientific test that can not only spot bubonic plague, but also every other pathogen present, according to the team working at Charterhouse Square. The research confirmed that Yersinia pestis was indeed behind the deadly pandemic. It also showed that 14th-century Londoners were already in poor health, suffering from rickets, anemia, and bad teeth, which made them ripe for the slaughter.
Although the plague travelled from one country to another via infected rats on board trade ships, and most likely attacked humans in the form of infected fleas, the fast-acting contagion could only have spread through contact between city dwellers, the London team concluded. Previous research showed that epidemic rates increased in the 14th century due to Europeans crowding the cities. Once the bubonic form found a foothold in a city, and made its way to the lungs of some inhabitants, the highly contagious pneumonic form of plague must have had the perfect breeding ground in overpopulated cities with inhabitants in poor health, like 14th century London.
Dr. Tim Brooks, an expert in infectious diseases at Public Health England, who was involved in the Charterhouse Square research, argued that bubonic plague alone could not have spread as fast from person to person, engulfing entire cities in a short amount of time. But the more virulent, airborne version of the disease could explain the deadly power of the medieval pandemic.
Fleas, however, may not be off the hook. Dr. Ken Gage, chief of the plague section at the Centers for Disease Control and Prevention (CDC), says the dissemination of the plague bacterium by fleas remains a valid theory for fast-acting epidemics. Gage agreed that rat-borne plague has a low risk of infection from human contact. What's more, there is not enough evidence to suggest that black rats, which typically carried the disease, were present in the British Isles, and no records of an explosive rat mortality, which should have preceded the human epidemic.
"The problem is to jump to the conclusion that only rats can be the vectors," Gage said in an interview. "In the British Isles, there could have been another vector, the human flea. We found that a lot of fleas don't get blocked very efficiently, but can transmit very efficiently in the first three to four days when they get the infection from an animal or even a human. If the human flea can do that, it could explain the spread of plague, which was most likely flea-borne, not airborne." Although some cases could have been pneumonic, Black Death records identify the disease by swollen lymph nodes, a characteristic of bubonic plague. Infected people can develop a secondary, pneumonic plague, which can be airborne, but in primary pneumonic cases, people would not have lived long enough to develop buboes, Gage explained.
The Public Health England expert agrees that fleas and rats were part of the process. "Infected fleas can stick around for some weeks, waiting for a victim, to whom they transmit the infection," Brooks explained. "This person will develop bubonic plague. Ten percent of people who have bubonic plague will develop pneumonia from the systemic spread, and in their dying hours will cough this up. People within 2 meters or less of the victim are at risk of infection through inhalation. They develop pneumonic plague, which can be transmitted to others as droplets. So if you have enough people who can be infected by fleas, you will have a good chance of getting some pneumonic cases. If these people live ten to a 3 meter by 3 meter room, they will get the disease as pneumonic plague, as will those who come to care for the dying family. They take it home to their loved ones and so on. Therefore, in the right social conditions, with the right circumstances to bring humans and rats together, and in a society where caring for relatives and neighbors was the thing, you get bubonic plague, followed by pneumonic plague."
A Channel 4 documentary that aired Sunday, April 6, explores the new archaeological findings and Brooks' theory.
Experts have yet to agree on the real culprit behind human plague pandemics. But whether people get the disease from fleas, infected animals, or from each other, once it becomes airborne, it can generate a public health crisis.
Plague is now treated with antibiotics, and can be prevented by applying insecticides to eliminate infected fleas. However, it remains a disease with a high mortality rate if left untreated. Since the plague bacterium has been used as a biological weapon in the past, with medieval armies catapulting corpses over city walls and modern armies suspected of dropping infected fleas from airplanes, concerns that the plague could fuel bio-terrorist attacks remain valid in our times. Plague vaccines are in development but are not expected to be commercially available in the immediate future.
Prevention guidelines from the CDC include reducing rodent habitats around homes, using flea repellents on skin and clothing, and applying flea control products to pets. While these measures may prevent an outbreak, focus should shift to human carriers once plague cases are detected in a community. One thing we can take away from the Charterhouse skeletons is that chain transmission among humans can have catastrophic consequences.
Close face to face contact—within two yards of each other—is needed to transmit the infection among humans, Gage said. Although bubonic cases still predominate today, chain transmission after the plague becomes pneumonic is the real challenge, as illustrated by the Madagascar outbreaks.
But the good news is that we can interrupt the chain very quickly nowadays, the CDC specialist said. We can treat the people who get infected, give prophylactic antibiotics to protect those who come in contact with plague patients, wear surgical masks, and isolate hospital staff and others who care for patients.
"Plague has everything it requires to cause the Black Death," Brooks said. "All it needs is the opportunity. It’s still there. It’s still the one that we used to have, it still has all the power and the threat that it used to have, and it’s only a different set of circumstances that’s keeping it in its place. Plague has what it takes. Plague can do it again."