In human history the bubonic plague (Y. pests) has come in 3 waves. In 542 CE the Plague of Justinian spread through the Byzantine Empire. Several hundred years later in 1347 CE the Black Death ravaged much of Asia and Europe. Most recently, the Modern Plague spread across international shipping lanes and killed 12 million people. All three times the spread of the disease was facilitated by trade routes. Justinian united the Mediterranean basin, the mongols conquered Asia and re-established the Silk Road, and the modern plague was transmitted across international shipping lanes. Although the plague still exists and still infects individuals it has not risen to its peak virulence since the Middle Ages despite having an almost identical genetic makeup.

Several factors contribute to the plagues diminished virulence. Firstly in the Middle Ages cities were crowded and there was no modern sanitation. Trash and fecal matter filled the streets and rats living in the waste could carry fleas that transmit the disease. Popular belief led people to believe that cats carried the disease since they were susceptible to the plague. When towns killed all of their cats rat and other rodent populations increased allowing for greater flea populations. In the modern plague scientists new to exterminate local rodent populations as they are a vector of the disease but in the Middle Ages and Byzantine Empire people thought it was the wrath of God, blamed immigrants, merchants, and the Jews. These epidemics led to some of the first quarantines and to early attempts to understand the disease and curative agents rather than look to scripture for a cure.

The plague also has the power to reshape culture after it has run its course. After surviving the plague emperor Justinian erected many churches as well as changed parades from a greco/roman style to a christian style. The Black Death however, had the opposite effect. Many christians lost faith with the church as their gospel could not save them, nor would many priests administer last rights. After the plague of the Middle Ages many christians turned to more private religious practices, and in the centuries that follow Europeans divided into many different sects of christianity. While the Justinian plague strengthened the church, the Black Death diminished the church’s power.

Today the plague does not bear the weight in society that it once did. If caught early enough it can be treated and although it is now endemic to the south western United States, and their is an ongoing outbreak in Madagascar, modern medical practices have kept it from becoming as destructive as it once was. Knowledge of incubation periods allow for quarantines of suspected infected individuals that last 3 days instead of 40 used in the past. Specific targeting of rodents rather than ethnic minorities allows for accurate targeting of disease carrying agents. Antibiotics and sanitation practices can cure patients as well as prevent its spread from host to healthcare worker to a new host.

Around 9,000 BCE humans discovered agriculture, and between the years 8,000 BCE – 6,000 BCE began the practice of domesticating cattle, sheep, and goats. This meant that for the first time in human history we could afford to live in one location instead of migrating as hunter gatherers. Agriculture led to increased food supply which meant that humans could have more children and live in larger communities. By 3100 BCE the Mesopotamian and Egyptian civilizations had developed and with them the firsts plagues also began to arise. Schistosomiasis spread in irrigation ditches. The disease is spread by flatworms that are naturally found in snails. These snails lived in the irrigation ditches that the Egyptians and Mesopotamians would use to irrigate their crops.

Fast forward several thousand years and trade routes have become routes of cultural exchange, trade, and a pathway for plague to spread across the globe. In 542 CE the first wave of bubonic plague had spread from China to the Byzantinian empire resulting in 25million deaths, and 50 million in the next two centuries. The disease crippled the empires economy and army, eventually leading to the destruction of the empire. 800 years later the disease re-emerged following the same pathway as before. This time the pandemic was the result of Mongolian invasion of an Italian trading outpost. In 1346 an outbreak of the plague broke out among the Mongols, which then spread to the Italian traders who spread it to the European continent, leading to the death of 1/3 of the human population. Some historians believe this outbreak made society as a whole more prone to violence as the mass deaths cheapened the value of life and lead to a period of wars and violence throughout the European continent.

The discovery of the New World and the resulting Columbian Exchange lead to the exchange of people, goods, animals, and microbes. Small pox, measles, plague, and malaria all made their way over to the new world, killing an estimated 90% of the indigenous population. Syphilis made its way back to Europe, carried by sailors returning from the America’s.

Today plagues are no where near as devastating as they wee in the past. Dedicated health care agencies like the CDC and WHO track outbreaks nationally and worldwide. They serve to monitor, prevent, and control the spread of disease. More importantly the discovery of antiseptics in the first half of the 20th century, and then the widespread use of vaccines in the second half of the 20th century have made entire populations of humans immune to specific diseases, and have eliminated incidence of small pox.

http://www.historyworld.net/wrldhis/PlainTextHistories.asp?historyid=ab25
https://en.wikipedia.org/wiki/Schistosomiasis#History
https://en.wikipedia.org/wiki/Bubonic_plague#History
http://www.who.int/diseasecontrol_emergencies/en/