Plague has a remarkable place in history and has had enormous effects on the development of modern civilization. Some scholars have even suggested that the collapse of the Roman Empire may be linked to the spread of plague by Roman soldiers returning home from battle in the Persian Gulf in 165 AD. For centuries, plague represented disaster for people living in Asia, Africa and Europe and because the cause of plague was unknown, plague outbreaks contributed to massive panic in cities and countries where it appeared.
Numerous references in art, literature and monuments attest to the horrors and devastation of past plague epidemics. We now know that plague is caused by a bacterium called Yersinia pestis that often infects small rodents (like rats, mice, and squirrels) and is usually transmitted to humans through the bite of an infected flea. In the past, black rats were the most commonly infected animals and hungry rat fleas would jump from their recently-dead rat hosts to humans, looking for a blood meal. Pneumonic plague, a particular form of plague infection, is instead transmitted through infected droplets in a sick person's cough.
Three Major Plague Pandemics
The Justinian Plague
The first recorded pandemic, the Justinian Plague, was named after the 6th century Byzantine emperor Justinian I. The Justinian Plague began in 541 AD and was followed by frequent outbreaks over the next two hundred years that eventually killed over 25 million people (Rosen, 2007) and affected much of the Mediterranean basin--virtually all of the known world at that time.
"Black Death" or the Great Plague
The second pandemic, widely known as the "Black Death" or the Great Plague, originated in China in 1334 and spread along the great trade routes to Constantinople and then to Europe, where it claimed an estimated 60% of the European population (Benedictow, 2008). Entire towns were wiped out. Some contemporary historians report that on occasion,there were not enough surviviors remaining to bury the dead (Gross, 1995). Despite the vast devastation caused by this pandemic, however, massive labor shortages due to high mortality rates sped up the development of many economic, social, and technical modernizations (Benedictow, 2008). It has even been considered a factor in the emergence of the Renaissance in the late 14th century.
The third pandemic, the Modern Plague, began in China in the 1860s and appeared in Hong Kong by 1894. Over the next 20 years, it spread to port cities around the world by rats on steamships. The pandemic caused approximately 10 million deaths (Khan, 2004). During this last pandemic, scientists identified the causative agent as a bacterium and determined that plague is spread by infectious flea bites. Rat-associated plague was soon brought under control in most urban areas, but the infection easily spread to local populations of ground squirrels and other small mammals in the Americas, Africa, and Asia. These new species of carriers have allowed plague to become endemic in many rural areas, including the western U.S.
However, as a bacterial disease, plague can be treated with antibiotics, and can be prevented from spreading by prompt identification, treatment and management of human cases. Applications of effective insecticides to control the flea vectors also provide assistance in controlling plague.
The most recent plague epidemics have been reported in India during the first half of the 20th century, and in Vietnam during wartime in the 1960s and 1970s. Plague is now commonly found in sub-Saharan Africa and Madagascar, areas which now account for over 95% of reported cases (Stenseth, 2008).
Plague as a Weapon of War
As a highly contagious disease with an extremely high mortality rate if left untreated, Yersinia pestis has been used as a weapon of biological warfare for centuries. Some warfare strategies have included catapulting corpses over city walls, dropping infected fleas from airplanes, and aerosolizing the bacteria during the Cold War (Stenseth, 2008). More recently, plague raised concern as an important national security threat because of its potential for use by terrorists.