The public health threat of emerging viral disease.

"Emerging diseases" are those that either have newly appeared in the population or are rapidly increasing their incidence or expanding their geographic range. Emerging viruses usually have identifiable sources, often existing viruses of animals or humans that have been given opportunities to infect new host populations ("viral traffic"). Environmental and social changes, frequently the result of human activities, can accelerate viral traffic, with consequent increases in disease emergence. Host factors, including nutrition, have often received less attention in the past but are of considerable importance. These factors, combined with the ongoing evolution of viral and microbial variants, make it likely that emerging infections will continue to appear and probably increase, emphasizing the need for effective surveillance.