The RNA viruses that cause disease in humans are a remarkably diverse group of microbes with a variety of morphological and genetic adaptations and extreme and novel biological characteristics. Viruses are assigned to one of 12 families on the basis of their envelope, capsid, and the nature of their RNA genome. RNA viruses are etiologic agents in a number of serious and prevalent human diseases, including influenza, AIDS, hepatitis, and viral encephalitis. Many experts believe that these viruses and dozens of others have the potential of becoming the most important disease threats of the future.