his diary. Daniel Defoe later recalled his experiences. For their own safety, both men left London as the death toll mounted. What was that plague that killed thousands and brought commerce to a stand still? Consider the evolution of humans: small migrating clans of hunter-gatherers, then on to established agrarian societies of mixed occupations and cottage industries and finally, to where we are today – all of the above plus large populations packed into well-connected towns and cities supported by colossal industries. Nature has a way of keeping populations in check by providing an effective limiting factor - parasites! One of the earliest and most dramatically destructive parasites to affect human population was a bacterium whose natural host is not human, but a flea. Yersinia pestis is a bacterium that attacks the bodies of Xenophylla cheopis, more commonly known as the oriental rat flea. CDC photos of Yersinia pestis bacteria on the left and Xenophylla cheopis - oriental rat flea on the right Rattus rattus - the black rat. Photo Credit The flea suffers horribly from this bacterial infection. The bacteria reproduce inside the flea to such numbers as to bung-up the flea’s stomach. The flea begins to starve. In an effort to satisfy its hunger, it continually bites the rat to no avail and ends up regurgitating on the rat's open wound. Ah! Flea vomit! Multiple flea bites thoroughly mixed with Yersinia pestis! Humans become infected through fleabites or by coming in contact with body fluids of an animal (including another person) that has the infection. There are three types of plague, all caused by the same bacteria. Bubonic plague shows symptoms called buboes that are swollen lymph nodes full of bacteria. During a bacterial infection, a body’s defense system reacts by sending macrophages, like white blood cells, to surround and destroy the bacteria. A symptom of this battle is swollen lymph glands under the chin, armpits, and groin. However, Yersinia pestis has a mechanism to resist destruction. So, instead of dying and being absorbed, the bacteria kill the macrophages and continue to multiply causing the nodes to become engorged and inflamed. This type of plague is not highly contagious but if left untreated it can progress to the more advanced and transmittable forms. When the bacteria pass into the blood stream and affects the major organs, it causes septicemic plague. The early symptoms of septic shock, caused by the bacteria releasing endo-toxins into the blood, are a bit non-specific with high fever, chills, nausea, and vomiting (so-called flu-like symptoms). This may have been why the Bills of Mortality, a kind of obituary notice, in Daniel Defoe’s account seemed so vague and often attributed death to distemper. Later symptoms include low blood pressure and internal bleeding. Blockage of arteries and the internal bleeding causes the skin to bruise and turn black as the cells die. This type of plague can be contagious if a person comes in contact with infected body fluids. Pneumonic plague is highly contagious and may have been responsible for spreading the plague so rapidly in the Three Great Plagues. In patients with pneumonic plague the bacteria has infected the lungs and can survive in even the finest droplets emitted by the patient. Without treatment, this form of plague was extremely deadly. Even with today's medicines, the survival rate of pneumonic plague is extremely low. As mentioned above, there have been three great plague pandemics in recorded history and although the epidemic of 1665 killed over 70,000 people, it is not one of three. There may have been other large pandemics, but there is no written account to support that. Most pandemics occur when humans have somehow tipped nature's balance through war, famine, or over-crowding. The first pandemic, called the Justinian, started in the Byzantine capital of Constantinople in 541-542 AD and lingered on in Eastern Europe until the 700’s. Under Justinian I, the population of Constantinople increased to the point that to feed its people the city had to import grain from Egypt. The Egyptians enjoyed several years of bumper crops and their vast granaries were overflowing. The rats took advantage of the surplus food and bred in great numbers, making our old friend Yersinia pestis very happy. It is believed that the plague rats, along with their infected fleas, were imported to Constantinople in ships carrying grain from Egypt. An estimated 40% of the people in Constantinople died in that outbreak. The second pandemic is the one most of us studied in our history books and occurred between 1348 and 1350. It was called The Black Death. Some people think the word “black” came from the symptom of black skin while others believe the word is just synonymous with gloom and doom and represents an appropriate adjective. This particular pandemic may have started in 1331 in China when plague killed about 25 million Chinese and Indians. Yersinia pestis probably made its way along the Silk Road reaching the Crimea in 1346. Previously it was thought that fleas hitched a ride in the fur of black rats that populated the merchant ships of the Mediterranean. But recent DNA analysis on the skeletons of victims reveals that the outbreak was probably the pneumonic form and spread from human to human. Watch how fast the pandemic crossed Europe in this animated map. The Black Death occurred during the Dark Ages when accurate accounts were rare. Casualty numbers varied widely but it is estimated that 30-60% of the European population died during that pandemic. Even when considering the conservative number, that loss must have made a significant global impact. Ever since “the big one” in the 14th century, there have been other outbreaks, including the one featured in Taken Aback. However, they don’t count as part of the major three. Just like an earthquake has aftershocks, plague flairs up and causes occasional epidemics. Started in China in 1855, just 150+ years ago, the third pandemic hit every continent and it wasn’t until the late 1950’s that the U.N.’s World Health Organization downgraded this pandemic from its active phase. Over 12 million died in India alone and the majority of those cases were bubonic. The strain of bacteria that caused the third pandemic reached San Francisco, California in 1900. It was during this pandemic that Alexandre Yersin, in 1894, identified the bacterium responsible for the disease and in 1898, Paul-Louis Simond discovered the role of the flea as a vector. The third pandemic is over, but Yersinia pestis is still out there – waiting.