The Zika virus probably entered the Americas between May and December 2013, more than a year before it was first detected in Brazil.

A team led by Pedro Vasconcelos at Brazil's Ministry of Health in Ananindeua sequenced the genomes of seven Zika viruses isolated from patients and linked to the current outbreak of the disease in Brazil. They found little diversity among the viruses, indicating that they are all related to a single root virus that entered the country in 2013.

At that time, a growing number of passengers were arriving in Brazil by air from Zika-endemic areas such as the Pacific Islands, which experienced Zika outbreaks between 2012 and 2014.

Science http://dx.doi.org/10.1126/science.aaf5036 (2016)