We live in an era of rapidly changing global landscapes and local environments. Viruses with RNA as their genetic material can quickly adapt to and exploit these varying conditions because of the high error rates of the virus enzymes (polymerases) that replicate their genomes. It comes as no surprise, then, that several prominent recent examples of emerging or re-emerging diseases are caused by RNA viruses. However, a complex interplay of factors can influence disease emergence. In addition to virus genetic variation (mutation, recombination, and reassortment), environmental factors (including ecological, social, health care, and behavioral influences) can play important roles. These can include (i) changing weather patterns (e.g., El Nino effects) and damming of rivers, which alters potential virus vector or host abundance and distribution, and (ii) tropical deforestation, which brings humans in close contact with these species-rich (hosts and their parasites) environments. Such factors, coupled with enormous increases in the human population during the last 50 years and urbanization in many developing countries, have greatly expanded the number of sampling events testing the fitness of RNA virus variants in different human cell backgrounds and potential transmission modes. This change, together with the advances in the speed and volume of global transportation, combines to create increased opportunity for emergence and re-emergence of viral diseases. The purpose of this review is to present some prominent recent examples of emerging and re-emerging RNA virus diseases (influenza, hantaviruses, Ebola virus, and Nipah virus) to try to convey a sense of the excitement within this field and the important advances coming about as new technologies are being applied to research the basic question of how new disease outbreaks occur and whether we can gain predictive capability.