The Zika virus has probably been circulating quietly and undetected in Brazil since 2013, a new genetic study shows.
A large international team of experts used a "genetic clock" to show Zika virus has changed about as much as would be expected if it had been carried into the country in 2013, NBC News reports.
They used samples taken of Zika cases from across Brazil and looked for mutations in the genetic sequence. The Zika now spreading explosively across Brazil, other parts of the Americas and the Caribbean very closely matches a strain that circulated in French Polynesia in 2013, the team reports in the journal Science.
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"We estimate that the date of the most common ancestor of all Brazilian genomes is August 2013 to April 2014," they write. The most likely date: December 2013. Brazilian officials first noticed Zika spreading in May of 2015.