The term emerging virus was coined by scientists in the 1990s to describe the agent of a new or previously unrecognized infection. The term implies that emerging viruses are new; however this assumption is incorrect. New virus infections have been emerging for thousands of years, at least since the rise of agriculture 11,000 years ago. The development of agriculture and commerce provided the large populations needed to sustain human infections such as measles and smallpox.
Viruses probably (although we do not know for sure) appeared when living cells evolved, possibly even before. They subsequently infected multicellular forms of life and then mammals, which were present on the globe before humans. Humans then acquired virus infections from animals (an infection transmitted from animals to humans is a zoonosis). At some point the number of virus genomes and virion structures became established, and for the next millions of years, viruses evolved. It is unlikely that new viruses emerge de novo; rather they evolve from existing viruses.
Some examples serve to illustrate the origins of viruses. Comparisons of genome sequences of today’s members of the herpesvirus family has lead to the suggestion that these viruses arose 180-220 milllion years ago, possibly from ancestors of similar viruses that infect oysters and fish today. Smallpox virus may have emerged after an infection of humans with a gerbil poxvirus. Measles virus may have originated from infection of humans with an ancestor of a virus that today infects cows, rinderpest virus. It has been suggested that the virus ‘jumped’ from cows to humans about 5,000 years ago, when humans first began to domesticate cattle. Measles virus then spread throughout the Middle East and was then brought to the Americas by colonization and migration, where it had lethal effects on the Native Americans.
It is safe to say that all of the human viruses that exist today originated from a zoonotic infection. In some cases, related viruses still infect animals (e.g. measles and rinderpest virus). However, often the human virus has no known counterpart in animals. An example is the human pathogen, poliovirus. The ancestral poliovirus is not known, and there are currently no hosts for the virus other than humans. However, other members of the picornavirus family, of which poliovirus is a member, infect a variety of animals, and ancient versions of these viruses may have made the jump from animals to humans.
Two simple facts ensure that new human virus infections will continue to emerge from animal hosts. The first is the ability of viruses to produce huge numbers of progeny (billions and billions!) with a high level of diversity (mutation). The second is the fact that human and non-human animal populations continue to grow and interact. Put another way, humans are always finding new ways to acquire novel virus infections!
Viral evolution is a fascinating subject. A good place to start reading about it would be Principles of Virology, volume II, chapter 10.