Scientists studying the Zika outbreak in Brazil say previous exposure to dengue, another mosquito-borne virus, may exacerbate the potency of infection.
New research published in the journal Nature Immunology suggests antibodies the body develops to neutralize dengue can mistake Zika for more of the same. This could allow Zika to enter human cells without triggering an effective reaction from the body's immune system.
This phenomenon has already been observed with dengue and could explain why getting dengue fever for a second time often results in a more severe infection if the virus comes from even a slightly different strain.
"This may be why the current outbreak [of Zika] has been so severe, and why it has been in areas where dengue is prevalent," said Gavin Screaton, a professor at Britain's Imperial College who led the research.
Dengue infections have increased dramatically over recent decades — to the point that the virus now causes an estimated 390 million infections each year around the world.
Zika, which is spread by the same mosquito as dengue, began causing alarm late last year when it was linked to a sudden spike in the number of babies born in Brazil with a developmental problem known as microcephaly, which is characterized by abnormally small heads. The virus has since spread rapidly throughout the Americas.
"We know that Zika has been present in Southeast Asia and Africa for many years and yet has not taken off there as it has in South America," says Jeremy Farrar, infectious disease specialist and director of the health charity which part-funded the research. "This is what the international research effort needs to work out, and quickly."
While the scientists are seeking to understand why Zika became so virulent and potentially dangerous in Latin America, ordinary people are already adapting their lives to its presence.
According to another study published this week in the New England Journal of Medicine, there's been a big spike in the number of pregnant women across the region who are seeking access to abortion pills online.
As the virus spread, several Latin American countries advised women to avoid pregnancy, but they did not offer access to birth control or abortion. The World Health Organization also advised couples living in areas with Zika transmission to consider delaying pregnancy.
Researchers compared requests for abortion pills made before and after November 2015, the date of the first warnings of a link between Zika and birth abnormalities. They found an increase in requests in seven out of eight countries where Zika is circulating. Venezuela and Ecuador top the list, with a rise of 93 and 108 percent respectively.
Abortion pills are only offered in the first trimester of pregnancy, but signs of microcephaly rarely appear until the second trimester, when abortion is illegal in most places.
Dr. Thomas Gellhaus, president of the American Congress of Obstetricians and Gynecologists, said the study underscores the need for legal abortion.
"The Zika crisis makes it impossible to ignore that women around the world do not have access to this basic health care need," he said.
Follow VICE News on Twitter: @vicenews