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By Maggie Fox

The Zika virus was circulating in Haiti in 2014, long before it became obvious that it was spreading fast in Brazil, a new study finds.

A team that went back and checked out three mysterious infections in Haiti found they were caused by the Zika virus. Their study raises questions about when and how Zika actually arrived in the Americas.

A military truck carries out fumigation in a neighborhood to stop the breeding of the dengue mosquito in Havana March 1, 2016.ENRIQUE DE LA OSA / Reuters

"We know that the virus was present in Haiti in December of 2014," said Dr. Glenn Morris, a professor of medicine and the director of the University of Florida’s Emerging Pathogens Institute. "And, based on molecular studies, it may have been present in Haiti even before that date."

Zika’s sweeping across the Americas and the Caribbean, as well as parts of the South Pacific. Once virtually ignored as a harmless and unremarkable virus, it is now alarming doctors and health officials because it’s causing devastating birth defects and paralyzing neurological conditions.

Brazil sounded the alert last year and at first scientists thought it had arrived there in 2014.

Related: Zika Mysteries Stump Even the Experts

But earlier this year, 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. And it very closely matches a strain that circulated in French Polynesia in 2013.

What’s not clear is why it’s now being seen to cause disease. Tests show it has mutated a bit, as all viruses do, but it’s not yet clear if the mutations somehow make it more virulent. It’s possible that because it’s hit a very large population with no previous immunity to Zika, what would have been events so rare as to go unnoticed are now getting attention.

"We know that the virus was present in Haiti in December of 2014."

Zika is a close relative of dengue and chikungunya viruses, which have been spreading for longer. And it is spread by the same Aedes aegypti mosquito. But Zika is usually milder than dengue and even than chikungunya, not even causing noticeable symptoms in most people and usually causing little more than a rash, fever and body aches.

Blood tests often confuse the three viruses.

Writing in the Public Library of Sciences journal PLOS Neglected Tropical Diseases, the team said they went back and tested blood samples taken when chikungunya was sweeping across Haiti.

“Zika virus was identified in plasma from three students seen in the Christianville Foundation Schools clinic,” the team wrote. They included a 15-year old boy, a 7-year-old girl and a 4-year-old boy.

None was very sick at all. None had the rash that so clearly characterizes Zika.

“In none of the cases would it have been possible to have identified the illness as a Zika virus infection based on clinical presentation, rather than dengue virus or chikungunya virus,” they wrote. So at least some of the cases diagnosed as chikungunya may have been Zika instead.

"There is a possibility that this virus had been moving around the Caribbean before it hit the right combination of conditions in Brazil and took off."

Because the children came from different schools and different towns, this suggests Zika was already fairly widespread in the area.

“Officially, no cases of Zika virus infection were reported by the Haitian Ministry of Public Health and Population until January 6, 2016, when five cases were confirmed in patients in the metropolitan Port-au-Prince area,” the researchers wrote.

Related: New Map Finds 2 Billion at Risk of Zika

But no one was really looking for Zika in the Caribbean until after the U.S. Centers for Disease Control and Prevention issued a travel alert in January of this year.

"There is a possibility that this virus had been moving around the Caribbean before it hit the right combination of conditions in Brazil and took off," Morris said.

The Aedes aegypti and Aedes albopictus mosquitoes that spread zika and chikungunya are found across the southern United States and as far north as New York.

The Pan American Health Organization says Mexican researchers have found Zika in A. albopictus. If this becomes common, it suggests even wider spread of Zika than predicted, because these cousins of Aedes aegypti live farther north and circulate for longer.

Haiti has almost the perfect conditions for the spread of mosquito-borne illnesses such as Zika. It’s got a warm climate, people are poor and live in homes without screens or air conditioning, and there are plenty of places for mosquitoes to breed.

Experts expect much less spread in the U.S., where most people live behind closed doors, with screens and air conditioning.