Sylvatic transmission cycles of yellow fever continue to occur in endemic tropical regions. If the outbreak in Brazil leads to urban spread, U.S. clinicians should adopt a high index of suspicion for the disease, particularly in travelers returning from affected areas. Four arthropod-borne viruses (arboviruses) have recently emerged or reemerged in the Americas, spreading rapidly through populations that had not previously been exposed to them and causing substantial morbidity and mortality. 1 The first was dengue, which reemerged to cause widespread disease predominantly in South America and the Caribbean in the 1990s. This epidemic was followed by West Nile virus in 1999, which has since become endemic in the continental United States, and chikungunya in 2013, which continues to cause disease, predominantly in the Caribbean and South America. Most recently, Zika virus emerged in Brazil in 2015 and spread through infected travelers . . .