Although first discovered in 1947, Zika virus (ZIKV) had not until recently been a major public health concern. However, since its arrival in Latin America, which has been dated by genetic analysis to 2013 but was first reported in 2015, a rapid regional spread of ZIKV outbreaks has been associated with microcephaly in infants and a rare autoimmune condition of the peripheral nervous system in adults. Hay and colleagues modelled the regions that are environmentally permissive for the transmission of ZIKV. In addition to regions that have ongoing outbreaks of ZIKV, the models predicted that many other regions, such as southeastern states of the United States, parts of the Indian subcontinent, sub-Saharan Africa, southeast China and northern Australia, may be at risk. Collectively the at-risk regions are home to 2.17 billion people.