Emergence of new viral infections: implications for the blood supply

New viral infections of humans appear to be emerging at an increasing tempo, probably due to technological developments and societal changes that have enhanced the ability of viruses to invade and spread within the human population. Emergence can be due to several discrete phenomena. Existing viruses of humans continue to be isolated, identified, followed by the description of new nosological entities. An increase in the ratio of cases:infections for ubiquitous viruses may lead to the emergence of new epidemics, associated either with the altered host susceptibility or increased viral virulence. Finally, viruses may invade populations from which they have disappeared or may cross the species barrier to invade new species. When a virus invades a new species it usually fails to spread from individual to individual. However, on rare occasions a virus adapts to a new species, usually involving mutations in the viral genome that alter the cellular host range. In such instances, if circumstances favor transmission, a pandemic may occur. For a new virus to endanger blood products, several criteria must be met. First the virus must be able to spread widely in the human population; second, it must cause a persistent plasma viraemia of moderate to high titre (although there are exceptions); and third, virus carriers must be asymptomatic. This set of conditions has only occurred once in the recorded history of human virology, with human immunodeficiency virus. It would probably be difficult to predict another invasion of the human population with a new virus, at least from studies in comparative virology. Probably the best deterrent strategy to facilitate the early recognition of any such hypothetical rare occurrence is the identification and control of all viruses that presently endanger the blood supply, supplemented by an active surveillance system to identify post-transfusion illnesses of any kind.