Chinese proverb claims that the rat is the most clever and strongest survivor of all animals. The rat proved itself in the beginning of time when god held a race for all of his animals; the rat was clever enough to hide in the ear of a large beast, and at the last second he leapt out of the beast's ear and crossed the finish line before the beast. Disease tells a similar history, in which it has been able to spread itself through out the entire world, through the naïve and unknowing help of humans and animals.
Earlier in history, Europe seemed to be an epicenter for disease. This was rooted in the fact that the Europeans, more so than any other population, lived intimately with their livestock. Farmers and the surrounding aggregate nations survived harsh winters and hot summers where it was necessary to share indoor space with animals. Additionally, they had excessive contact with them and little understanding of what hubs of nastiness their fury friends were or what control centers for disease they themselves would become. Humans have historically shown a deep affinity for livestock and continue to do so, currently, in New Zealand there are more sheep kept than there are people on the island. In fact, in some cases, people seemed to have developed such a tenderness for their animals that they resort to practicing bestiality. Several common human diseases are closely related to animal diseases. Smallpox developed from cowpox and measles is closely related to rinderpest (another bovine disease). But whether it is through cuddling, milking or whipping our animals, we have managed to remain intimately involved over the centuries; and disease has lovingly appreciated it by spreading itself far and wide.
The populations of the American continents however, did not experience the same spread of disease that the Europeans did. They kept almost no domestic animals. Consequently, their immune systems had few antibodies developed to combat even the weakest strings of disease. Also, they did not live in crowds the way that Europeans did. Early in the middle ages the Europeans had not only settled and become dependent on their agriculture and livestock, but they had developed large cities like London where thousands of people lived in close quarters, and had given little thought to the disposal of waste. Native Americans did live in tribes but did not have permanent living quarters that compared to the size of some of the European cities. In fact, most of the tribes that lived in the Midwest were migratory, moving for the winter season. Their constant movement, small numbers and lack of exposure to livestock made it difficult for any crowd disease to develop or have a devastating impact on their population. It is hypothesized however, that the Aztecs (who lived on the American continent) were possibly wiped out by crowd disease, whether this included exposure to Europeans or not is unclear, nothing can be known for sure.
Consequently, when the Europeans arrived in the America's, disease was their greatest weapon. Smallpox in particular had devastating effects on the indigenous people. Unfamiliar with the phenomenon of crowd diseases, the Native Americans knew none of the tricks that the Europeans had been quickly learning, like containment. During the Bubonic Plague which caused its first devastating outbreak in Europe in 1346, houses were nailed shut with victims in them and parts of cities were burned in an attempt to tame the ravaging disease.
Asia however, did not suffer the same fate that the Americas did. In fact, Asia is the continent that gave Europe the Bubonic Plague, or Black Death. It was spread by flees that hung onto rats that traveled from continent to continent in cargo ships. The extensive trade between Asia and Europe allowed us to spread almost all of our diseases over the spread of time, especially as the Silk Road became more developed and traveled.
The Asians had a much better understanding of sanitation than the Europeans during the Middle Ages. Asians bathed themselves on a regular basis and had a fairly strong foundation and understanding or herbal medicines. Not only did Europeans not bathe themselves on a regular basis, but they paid little attention to their drinking water, giving cholera an open door to spread itself. An example of poor sanitation directly resulting in death or illness is the hairstyle of European women in the sixteenth and seventeenth century. Women used to wear their hair on the top of their heads and then have wigs fitted over their hair, occasionally women developed such horrendous lice infestations that their heads became nests to other insects as well, including nats, which would sometimes burrow into their scalp, lay eggs and kill them.
World trade routes were the solidifying factor for the spread or microbes. Once Asia, Europe and North Africa had ban together they ultimately meshed their worlds into one giant breeding ground for bacteria. Agriculture continued to assist in the spread using animal and human feces as fertilizer. But at the same time, developments in medicine and better sanitation, along with better diets and eating habits, strengthened humans to fight the mass crowd diseases.
Europe however, was not only given the gift of disease. There were several strange circumstances that allowed them to flourish throughout the world. First off, they had more large mammals after the first ice age which they were able to domesticate and use for Africa etc. But most importantly, they had a monotheistic religion which allowed them to be self prophesized. Through this came the development and burst of capitalism, for which nothing could be stopped. The greediness and neediness that capitalism created allowed them to spread at any cost. And travel they did, European's made an effort to conquer the world. They developed an insatiable curiosity for the unknown, which extended far beyond the boundaries of their own reality, both geographically and mentally.
Through human travel, the populations of humans have been heavily affected and in turn have had effects on wild life and its surrounding environment. Disease, however, is the main reason that travel has had such a devastating impact.