One of the more popular hypotheses about how the Zika virus arrived in Brazil maintains that it came with tourists attending the 2014 World Cup.  But scientists are now discounting that because of a growing pile of evidence that shows it got there earlier.

A study published in the journal Science suggests that the virus arrived between May and December in 2013.  That is long before any illnesses were first detected in 2015; but more importantly, well before the World Cup or another major sporting event called the World Sprint Championship canoe race in 2014.

The British and Brazilian scientists studied seven samples of the Zika virus taken from across Brazil, and analyzed their genetic code.  The seven were similar enough to suggest coming from a common source.  But Zika is a virus that mutates rapidly, and the small variations between the samples allowed the researchers to reconstruct its family tree and estimate when their common ancestor arrived in Brazil - somewhere in mid-to-late 2013.  And it appears to be closely related to the Zika virus that was detected in an outbreak in French Polynesia in 2013.

Scientists want to know everything about Zika as they work to eradicate it.  Since its introduction, Brazil has logged nearly 30,000 cases of the virus.  It is believed to be the cause for an uptick in cases of microcephaly and congenital abnormalities in infants born to mothers with the disease.