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/