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 the ir geographic range. Emerging viruses usually have identifiable source s, often existing viruses of animals or humans that have been given op portunities to infect new host populations (''viral traffic''). Enviro nmental and social changes, frequently the result of human activities, can accelerate viral traffic, with consequent increases in disease em ergence. Host factors, including nutrition, have often received less a ttention 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 p robably increase, emphasizing the need for effective surveillance.