Before the current Zika outbreak in Latin America (particularly Brazil) and the Caribbean, the mosquito-borne virus suspected to cause birth defects and other complications showed up in French Polynesia. Some 20,000 people living on the group of islands in the South Pacific are believed to have been infected with Zika between 2013 and 2014, NPR reports. It was the largest outbreak to date of the virus first discovered in Uganda's Zika forest in 1947. Before that, in 2007, about 50 people were infected on Yap Island in Micronesia the New York Times reports. In French Polynesia, doctors first suspected an outbreak of dengue, Dr. Van-Mai Cao-Lormeau tells NPR. But, she says, once you've had dengue, you're not likely to get a mild form of it again. "We were suspicious of this being dengue because all these people already had dengue." Testing ultimately confirmed it was Zika.
"It spread very quickly," Cao-Lormeau says. Zika is suspected to be linked to the birth defect microcephaly, which inhibits the growth of a baby's head and brain. In a retroactive investigation, doctors in French Polynesia found several cases of "central nervous system malformations in either fetuses or babies," Cao-Lormeau tells NPR. During the outbreak, Tahiti began seeing more cases of Guillain-Barre syndrome, a neurological condition linked to Zika. Soon, the Zika outbreak in French Polynesia disappeared. There hasn't been a confirmed case of the virus there since April 2014. Researchers, per the Times, don't know why the Zika outbreak in Latin America seems more aggressive. It could be the denser population, Cao-Lormeau theorizes. Or, the Times writes, there is another possibility being considered: "The virus has mutated and gotten worse." (This study provides first evidence of Zika being transmitted from mother to child.)