The mosquito-borne Zika virus will likely spread across South, Central and North America, according to a new warning from the World Health Organization.
Zika virus is thought to be linked to a rare birth defect called microcephaly that gives newborns abnormally small heads. This condition often results in an underdeveloped brain and can even cause death.
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On Monday, officials in Arkansas confirmed a resident tested positive for the Zika virus. The person had recently returned from traveling out of the country.
The WHO says two reasons for the spread are the populations previously unexposed to the virus and the prevalence of Aedes mosquitoes, the main carriers.
"The role of Aedes mosquitoes in transmitting Zika is documented and well understood," the WHO said, but also warned of rare cases of transmission via sexual contact and blood.
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Canada and Chile are the only countries of the 55 that don't have Aedes mosquitoes, which could help keep the virus out of their borders.
Since November, CNN reports there have already been more than 4,000 reported cases of microcephaly in babies in Brazil, where the outbreak began; there were 146 cases there in 2014.