Image of oriental rat flea (Xenopsylla cheopis) infected with Yersinia pestis (the bacteria causing pneumonic and bubonic plague) depicts the disease at work within the flea's body. Image online, courtesy the National Institute of Allergy and Infectious Diseases, Laboratory of Zoonotic Pathogens.
CAUTION: THIS CHAPTER CONTAINS DESCRIPTIONS OF A DEADLY DISEASE
Growing inside the rat flea, plague bacteria blocks the flea's stomach and food tube. The flea becomes insatiably hungry. It goes on a feeding frenzy, jumping to new hosts who exhale carbon dioxide. New hosts like carbon dioxide-exhaling people.
When the starving flea bites its new human host, it typically does two things. It takes blood, of course, but it also squirts saliva, or partly digested blood, into the bite. If the flea is infected with plague bacteria, it will pass that infection on to its new host.
Once in the human body, the bacteria first causes flu-like symptoms and a high fever. Next it invades the lymph nodes. Those nodes, especially in the neck, armpits and inner thigh area, swell. During the great pestilence, people referred to a swollen node as a "bubo" - hence the name "Bubonic Plague."
As the disease progresses, the victim bleeds internally and produces bloody urine and bloody stools. Dried blood puddling under the skin forms black boils which appear all over the body - thus the name "Black Death" (which was coined in the 1800s). Everything that leaves the sick person's body smells revolting.
Today, if the disease is caught while in stage one (flu-like symptoms with high fever) or stage two (painful, ugly buboes), a person can be successfully treated with antibiotics. But when the bacteria invade a victim's lungs, causing respiratory failure and pneumonia, the disease is almost always fatal. Worse, this form of plague (called "pneumonic plague") spreads unbelievably fast when an infected person sneezes or coughs.
Given the magnitude of the illness and the way people lived in the 1340s, a disaster of unparalleled proportions was about to unfold. Anyone who caught the disease had an 80% chance that death would follow within two weeks. By the time the Black Death ended, nearly half the population between India and Iceland was dead.