New viral diseases. A real and potential problem without boundaries.

Viruses are the most mutable and rapidly evolving human parasites. Therefore, it is not surprising that new virus diseases, such as the acquired immunodeficiency syndrome, are emerging. However, these new viruses are relatively few in number, which reflects the constraints placed on viral evolution by the requirement for maintenance of virus structure and function and virus adaptation to restricted ecologic sites. Most "new" viral diseases, such as the hemorrhagic fevers, result from environmental disruptions that increase human contact with animals or insect vectors of old viruses. However, truly novel viruses with lethal potential or capacity for global spread have emerged in animals as a consequence of single-point mutations or genetic recombination between viruses. Given the mutability of viruses and the inevitability of environmental change, we must be prepared for the evolution of new virus diseases.