The desire to travel and explore developed among many early sedentary civilizations. This mobility provided these cultures with many advantages that have helped to advance their societies. Unfortunately, these movements are also responsible for the transmission of numerous diseases and their resulting adverse effects upon the inhabitants of the Earth. This essay seeks to outline the causes of epidemic diseases, explain their diffusion around the world, and explore why they are more harmful in certain societies.

During the First Great Transition, small mobile groups of hunter-gatherers began to adopt sedentary lifestyles. This was facilitated by the development of agriculture and the practice of animal domestication. Although the foundation of agriculture originated in the Mediterranean, it spread to Europe, the near East and eventually the rest of the populated world. With more efficient methods of food production, the population of these groups began to significantly increase. Domesticated animals were not only used as a supplementary food source (meat and milk), but also for providing animal power in labor-intensive activities (such as plowing).

The transition to a sedentary lifestyle caused a major decline in health in these growing societies as virulent and lethal diseases began to appear. "The major killers of humanity throughout recent history-smallpox, flu, tuberculosis, malaria, plague, measles, and cholera-are diseases that evolved from diseases of animals, even though most of the microbes responsible for our own epidemic illnesses are paradoxically now almost confined to humans."1 As early farmers began to live closer to and spend more time with livestock and pets, the germs from these animals were transferred to humans. Upon infecting humans, these microbes developed techniques to spread among the human population. These techniques included, transfer by saliva, diarrhea, and skin lesions. These microbes not only evolved to feed off the nutrients provided by the human body, but also developed methods to spread between potential victims.

While most diseases spread slowly but steadily to infect a few victims at a time, the increasing number of infectious agents created epidemic diseases. These diseases, like influenza, rapidly spread throughout the immediate population resulting in death or complete recovery in a relatively short amount of time. Those who recover from the illness become immune to the disease for a long period of time. Thus, when outbreaks of epidemic diseases occur, they quickly infect the populous killing or creating immunity within the victims. Without new hosts, the epidemic dies out until the immunity wears off and the local population is again infected with the disease. Other examples of epidemic diseases, measles, rubella, mumps, whooping cough, and smallpox along with influenza are extremely virulent and lethal. These "crowd" diseases would not be able to sustain themselves in the smaller groups of hunter-gatherers or swidden farmers.

Societies that withstood the various epidemic diseases continued to grow and began to interact with neighboring civilizations, often in the form of trade. As trade became more important to the differing and expanding cultures, it also provided a stimulus for urbanization. Thus, cities "first developed as a result of intensive trading in the eastern Mediterranean region."2 Despite the importance exchanging resources and cultural ideology, diseases were also spread by "the development of world trade routes, which by Roman times effectively joined the population of Europe, Asia, and North Africa into one giant breeding ground for microbes."3 Eventually, every epidemical disease within these regions was diffused to the entire Eurasian continent along caravan routes and in port cities.

"The development of market economies over the past couple of centuries provided great impetus to the acquisition of wealth."4 As a result, many societies began intense exploration programs. Places with high populations and limited resources in particular, tended to travel more. Unfortunately, "increasing contacts between societies that had developed in isolation in different parts of the world substantially changed the spread of diseases in the world."5 This is especially exemplified in the deleterious effects of European exploration in the New World. In this instance, Old World germs are credited with ravaging the indigenous populations of the Americas.

The increased lethality of these diseases in the New World can be explained during the last ice age. During this glaciation, the continents of the world experienced megafaunal extinctions of large animals. While several species were fortuitously saved from extinction in Europe, few large animals survived in the Americas. The "luck of the draw in domestication went to Eurasia, where there were more large animals with the appropriate characteristics than elsewhere."6 With fewer domesticated animals, diseases did not readily spread from animals to humans in America as they did in Europe. With little immunity or genetic resistance to epidemic diseases, these microbes easily devastated the population of the New World.

Another factor that aided the lethality of diseases against the inhabitants of America was the absence of major trade. The "three most densely populated American centers-the Andes, Mesoamerica, and the Mississippi Valley-never became connected by regular fast trade into the one huge breeding ground for microbes."7 It can also be argued that epidemic disease were more fatal during this time period as they had not yet evolved to allow their host to live longer and therefore increase the chance to distribute their offspring to more victims. Furthermore, American inhabitants were inundated with new diseases without the opportunity to develop a resistance to infection. Not only did Spanish conquistadors introduce numerous European diseases to the New World, but slave routes also introduced African diseases as well.

After adopting the sedentary lifestyle that is associated with agriculture, people began to keep and live much closer to domesticated animals. These animals carried microbes that evolved to infect the human population. Epidemics are the result of especially infectious and lethal diseases. As trade became a compelling force for the cultural evolution of many societies, they helped to spread diseases over the entire world. Due to an auspicious piece of luck, Europe, with many more domesticated animals and therefore epidemic diseases, was able to develop immunities that the populous of the Americas lacked.