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Emergent viruses is a concept based on the fact that through certain selection pressures, viruses can adapt and can emerge as a new disease/pathogenic strain with a new attribute facilitating its pathogenicity in a new field not normally associated with that specific virus. This includes viruses that are the cause of a disease which has notably increased in incidence; this is often a result of a wide variety of causes from both the influence of man and nature. Most emergent viruses can be categorized as zoonotic (an animal disease that can be transmitted to humans), this has the advantage of possibly having several natural reservoirs for the disease.

The most important factor in the development of an emergent disease, to humans, is the ability to pass from animal host to humans. There is little to no occurrence of spontaneous new virus species development, although the possible exception commonly cited is Ebola virus. Most often the virus, due to selection pressure for an animal version of the strain of disease to mutate and therefore adapt to the infection of human hosts.

- Population Movements
- Deforestation
- Irrigation
- Urbanization
- Increased long distance air travel
- Increased long distance air travel for livestock
- Migration

Poliomyelitis.
Known to have existed for centuries with little incidence. During the 19th century poliovirus became more prominent in populations across the world. Dense urban populations allowed the disease to propagate via close human to human contact. In addition to this, technological advances meant that traveling became more common throughout the population essentially leading to increased transmission of the virus. The situation became worse year upon year with increasing incidence of the disease worldwide. The issue was not resolved until the introduction of a poliovirus vaccine brought the situation under control. This example demonstrates how a disease can emerge in a population as a result of human influence. This examples also illustrates how an emergent virus can emerge and disappear by tackling the root cause. Poliovirus has been targeted for eradication by the WHO.

New emergent viruses are continually being discovered. Examples include: SARS and Avian flu H5N1

• MicrobiologyBytes: Emerging Viruses