The Agricultural Revolution drastically changed the way that humans interact with each other and the environment, and caused infectious diseases to have a much more profound impact on our population. A combination of human, animal, and environmental factors (the interaction of which is called the “One Health” concept) created more suitable environments for infectious diseases while the steady globalization of societies has brought existing diseases to new hosts with little or no immunity. This diaspora of plagues has directly affected the development of the human race, and diseases have killed on a global scale on multiple occasions.
Ironically, the massive death tolls of the most infamous infectious diseases (e.g. smallpox and the Black Death) were made possible by one of the most important developments for the success of the human race: the transition from a hunter-gatherer society to an agricultural one. After humans discovered agriculture we have lived increasingly sedentary lives with exponentially-growing populations. The drastic increase in our population has allowed diseases to infect more hosts, and the close contact between humans due to population growth has made person-to-person transmission of disease a more common occurrence. This problem was exacerbated throughout much of human history by an ignorance of how diseases survive and spread. Early societies often had nutritional deficiencies from a limited dietary spectrum, little to no personal hygiene standards, no understanding of sanitation or other disease-prevention methods, ineffective medical science (often based on religious beliefs or the alignment of the planets), and had no idea what a microorganism was. Additionally, these societies had poor living conditions (often dumping their own waste outside of their doors) which were perfect breeding-grounds for the microorganisms they did not yet know existed.
It was not only close contact with each other and scientific ignorance that spread diseases through these societies. Being agricultural in nature, it was a common practice to live side-by-side with farm animals which allowed the evolution of animal diseases into human ones (seen today in the emergence of hybrid schistosomes in the Senegal River Basin). Living with animals also exposed early humans to animal feces, and sharing their water-supply opened yet another door for waterborne diseases (such as Typhoid fever) to enter. The animals that they lived with also provided hosts for insect vectors which spread the diseases they carried to humans as well. As people interacted with their environment in more dramatic ways the diseases carried by insects became a greater problem. Deforestation decreased the abundance of animal prey for insect vectors which led to them spreading more diseases to human hosts, while irrigation created significantly more areas of standing water for insects to breed in.
The increased effect of diseases due to the combination of these factors caused populations to develop immunity or resistance to diseases that were endemic to their area. However, the steady increase of globalization brought people into contact with diseases that they had no immunity to. A large part of the success of European countries at taking control of their colonies in the Americas was due to the lack of immunity to European diseases by the native populations. Smallpox in particular nearly wiped out indigenous people to the point that a massive influx of disease-resistant slaves from Africa was required to provide labor for these colonies. Earlier, the Black Death spread across Asia to reach Europe due to international travel and trade and proceeded to decimate the European population.
Our modern world is not immune to the dangers of globalized diseases; the Spanish flu (brought home by soldiers returning from World War I) killed millions of people, and SARS originated in China and quickly became a global threat that was only contained due to international collaboration. The technological advances in international travel have led to a world where disease can spread across the globe and reach pandemic levels in a matter of hours or days which requires constant monitoring and reliable reporting of new infectious diseases to prevent. Many diseases remain without a cure, and new deadly diseases regularly present themselves. Thankfully, our modern world has allowed for the collaboration of the scientific community on vaccination, treatment, and curing of these diseases. Their ongoing efforts to combat the spread of infectious diseases are of the utmost importance to prevent another Black Death.