A fourteenth century flea caused more death and destruction than any military leader or foreign invader in all of European history. The flea bit rats and other rodents that were infected with a deadly bacillus, then migrated to human hosts; in biting the humans, the flea transferred infected blood to its new host. The rodents died. The humans died. The flea lived a long and happy life. .
This epidemic, known as the Black Death, spread in two ways. The flea continued biting rodents, people and livestock; at the same time, infected people coughed and sneezed, passing the bacteria to their neighbors. The disease transmitted directly from the flea took the form of bubonic plague, and had a very low survival rate. When transferred from human to human, it took the form of pneumonic plague, and was 100% fatal. Once symptoms appeared, victims usually died within four days.
The Black Death actually originated in China in the early 1330s, and moved west via Mongol invasion. In 1347, Tatar warriors invented germ warfare, by catapulting infected corpses into an Italian outpost on the Black Sea. Horrified by this swift-spreading killer, the Italians fled by ship, unaware that they were transporting the deadliest cargo in Europe's history.
This lethal invader arrived in a Europe that was urbanized, densely populated, and had developed elaborate land, sea and river trade routes. The plague spread like wildfire, and within two years had wiped out more than one-third of Europe's population. The losses were heaviest in the cities: in Florence, 45% to 75% of the citizens died in the first year, at Avignon 50%, and in Venice 60% died within 18 months. Anywhere people lived in close quarters, such as the monasteries, convents, schools and universities, death rates were also very high. While losses were less in the countryside, rural Europe suffered a more permanent damage.
Economically, the impact was almost as swift and devastating as the population loss.