Worldwide, in 1930, only six viruses were known to be maintained in cycles between animal hosts and arthropod vectors like mosquitoes, gnats, and ticks (Karabatsos, 1985). Only one of the recognized arboviruses (arthropod-borne viruses), yellow fever virus, caused disease in humans. The other five viruses were responsible for epizootics and major economic losses in domestic animals: bluetongue in sheep and cattle, Nairobi sheep disease, Louping ill in sheep, vesicular stomatitis in cattle, and African swine fever.

Later in the same decade, there was an explosion of newly emerged arthropod-borne diseases in North America. Western and eastern equine encephalomyelitis viruses caused major outbreaks with high case fatality rates in both equines and humans. St. Louis encephalitis virus was associated with more than 1,000 cases and 201 deaths in residents of Missouri. Subsequent research demonstrated that each of these viruses was maintained in a cycle dependent on mosquitoes and birds. When any of the viruses invaded the human population, an epidemic often ensued.

Since the 1930s, 86 additional arboviruses have been found in North America. Fortunately, only a few, such as the California encephalitis complex, Colorado tick fever, and the dengue fever viruses, have been consistently associated with human disease. However, all 86 viruses are distributed widely, and many have thus far been shown to cause only inapparent infections in humans. A shift in the virulence of the viruses or in human susceptibility could potentially alter the present equilibrium. Some experts warn that the arboviruses are "viruses looking for a human disease."

The threat of arboviral disease is not limited to North America. The 1985 International Catalogue of Arboviruses (Karabatsos, 1985) identified 504 arboviruses worldwide, 124 of which have been associated with a disease. It is of continuing concern that nonindigenous viruses might be introduced into the United States through travel and trade.

The rate of discovery of arboviruses reflects the intensity of the worldwide search by the Rockefeller Foundation, government agencies, and universities from 1950 to 1980. There has been a significant decrease in activity for such programs in recent years, as seen in the table below. Yet all the while, new arboviruses continue to be found whenever and wherever a search is made.