An emerging virus is a term applied to a newly discovered virus, one that is increasing in incidence or with the potential to increase in incidence. Many viruses fit into this definition. HIV is the clearest example of a previously unknown virus that has now produced one of the largest pandemics in history. Recent advances have occurred in the identification and understanding of new hantaviruses in the Americas, causing an acute respiratory disease. The possible causal role of human herpesvirus 8 in Kaposi's sarcoma has gained support, whereas that of a newly discovered flavivirus in causing hepatitis has not been confirmed. A major advance has been evidence showing that the bovine spongiform encephalopathy agent is almost certainly the cause of a new variant of Creutzfeldt-Jakob disease. Although new viruses are discovered almost yearly (e.g., Australian bat lyssavirus), other "older" viruses (e.g., dengue) are reemerging, infecting millions of people every year with significant mortality.