All of these infections are termed "zoonotic" diseases, which means that they can be transmitted between humans and animals. More than one half of all known infections are zoonotic, as are approximately 75% of "emerging" infections (those that have been newly discovered, are found in new geographic areas, or are increasing in numbers).
There are several reasons for this emergence. Sometimes, it is the result of increased surveillance for infections in particular geographic areas or high-risk populations, as was probably the case with H5N1, H7N9, and MERS-CoV. It may be a localized outbreak that spreads widely, leading to the recognition of a new pathogen, as with SARS-CoV. It could be a pathogen that has spread relatively silently in the human population, and is not recognized until it affects a group unique enough to draw the attention of physicians. This is what happened with HIV/AIDS in the early 1980s, when the then-unknown virus caused unusual infections in a specific group: homosexual men. Or, it could be a pathogen whose clinical symptoms are so extreme that they are impossible to ignore, such as outbreaks of the Ebola virus.
Global changes over the past 3 decades have allowed new infectious diseases, and particularly zoonotic illnesses, to emerge from their animal reservoirs. Changes in land use -- for example, clearance of forests or farmland, or human encroachment into areas that were previously inhabited mainly by wildlife -- are key drivers of emergence. As humans come into contact with native animal species, they also are exposed to the microbes carried by these animals, some of which can cause disease in humans.
These animals don't even have to be exotic. Change in land use in Connecticut was one of the drivers of Lyme disease emergence, when previously cleared farmland was reforested, deer populations boomed, and residences were built among these woody tick habitats.[4.5] Humans were consequently more likely to be bitten by deer ticks that had been infected by their first hosts: the white-footed mouse, which carries Borrelia burgdorferi, the bacterium that causes Lyme disease.
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Cite this: Tara C. Smith. Human-Animal Infections: A Primer for Clinicians - Medscape - Dec 16, 2013.