Agriculture was a natural outcome of human civilization. Civilization brought with it the permanent settlement of areas rich in resources. Years of living in a region compounded by an increasing population led to a reduction of animals and plants for food. Consequently, this forced people to rely less on traditional foraging and hunting practices. They had to domesticate and raise the animals and plants needed for sustenance. This produced a situation in which people were living in close contact with the domesticated animals. Immediately after the domestication of animals came the onset of previously unknown human ailments. Only recently have we discovered that these diseases came from the domesticated animals. For example,we now know that the common childhood disease pink eye (conjunctivitis) originated in cattle. It still causes a related eye infection in dairy and milk cows. The notorious disease smallpox, in addition to a host of other widespread human ailments, was at one time limited to animals. Even today, domesticated animals are the source of new human diseases. An article published in the January 2003 issue of the scientific journal Trends in Parasitology reported how an infectious protozoan called Leishmania passed from wild animals to dogs and then to humans.Another study in the same journal is investigating the probable spread of a cattle protozoan disease to dogs and possibly humans.
Along with civilization came the exploration of new land to offset the crowding caused by rapid population growth in ancient towns and metropolitan areas. Food shortages became commonplace as agriculture and natural resources could not keep up with the growing population’s needs. People migrated to new areas to exploit the untapped resources. New areas brought exposure to new microorganisms that inhabited the local animals and soils. For example, the colonization of Europe from Africa introduced people to flu (influenza), measles, and tuberculosis. The highly destructive Ebola virus and the human immunodeficiency virus (HIV), which causes acquired immune deficiency syndrome (AIDS), may relate to human settlement of remote forests in Africa. Various encephalitis viruses, some of which are currently plaguing the United States, are believed to come from living in close contact with birds of the forest. Exploration also became a way of introducing emergent diseases into new areas. For example, human migration introduced black plague to Europe through trade with Asia. Europeans brought their emergent diseases to the Americas, killing many of the indigenous peoples.
Areas of the Middle East and North America in ancient times had the same overpopulation problems of modern cities. Crime, overcrowded housing, pollution, and poverty abounded. With these problems came the appearance of new diseases. Overcrowding in cities creates two factors that encourage the development of emergent diseases. First, it allows the buildup of wastes and litter that attract insects and rodents. This puts people in regular contact with microorganisms living on the pests. These microorganisms can then spread to people, causing previously unknown diseases. Second, overcrowding makes it easier for diseases to spread from one person to another. This allowed new diseases to gain a foothold in the human population by being able to jump readily to uninfected people. In rural areas, the diseases could not spread and simply died out with the infected people. Exploration and migrations to other populated areas further spread diseases.
The following excerpt from the CRC’s Handbook ofMarine Mammal Medicine, published in 2001, reveals one avenue for creating new human diseases.
Reports of the transmission of disease from marine mammals to humans are scarce; however, as humans are increasingly in contact with marine mammals, the possibility of encountering new disease must be considered. Lack of reports in the literature may indicate lack of occurrence of disease, but may also reflect lack of recognition by physicians or failure to report for a variety of reasons.Until recently, only hunters and scientists were likely to have close physical contact with marine mammals, and the public’s exposure was limited to zoos or aquaria with animals behind barriers. However, in the last decade, human contact with marine mammals has changed so that a broader range of people are exposed to zoonoses.
Increased exposure to marine mammals is related to the increasing popularity of boating, scuba diving, surfing, and swimming. Improvement in zoos makes for better contact with humans and animals, because the animals are less confined and are closer to people. Ecotourism can also be to blame, because more and more people are taking vacations that bring them in contact with marine mammals. Thus, the stage is set for the better transmission of disease between people and marine mammals (zoonoses), making real the possibility of new emergent diseases. Early in human history, hunting may have predisposed people to emergent diseases, as it does today with conditions such as Lyme disease.
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