in epidemiology, a class of viruses that has long infected humans or animals but now has the opportunity to attain epidemic proportions due to human encroachment on tropic rainforests, increased international travel, burgeoning populations in less developed countries, and, possibly, mutations. Several viruses have been termed this way, including hemorrhagic viruses such as Ebola, Marburg, and Hantaan; the two rabieslike viruses Mokola and Duvenhage; rodent-borne Junin and Lassa viruses; and mosquito-borne dengue. Virologists speculate that the strain of HIV that causes AIDS may also fall into this category, having entered humans through contact with monkeys in central Africa, having possibly existed among monkey populations for some 50,000 years.