The Zika virus may be using dengue infection as a "Trojan horse", according to British researchers.
The two viruses are very similar – but some antibodies generated by dengue fever may amplify Zika infection, according to the research at Imperial College, London.
The findings, from laboratory studies, have been published in Nature Immunology.
Dengue fever affects some 390 million people a year and like Zika is transmitted by the Aedes mosquito.
Researchers found that antibodies generated against dengue can recognise and bind to the Zika virus.
This causes a process called antibody-dependent enhancement. When this happens antibodies cannot effectively block the virus – and instead shuttle the virus into immune cells. The virus then "hijacks" the cell to replicate more viral particles, the researchers say.
In a second study in Nature, the researchers identify an antibody, EDE1, that might neutralise Zika.
Researcher Professor Gavin Screaton said: "Although this work is at a very early stage, it suggests previous exposure to dengue virus may enhance Zika infection. This may be why the current outbreak has been so severe, and why it has been in areas where dengue is prevalent.
"We now need further studies to confirm these findings, and to progress towards a vaccine."
Dr Jeremy Farrar, from the Wellcome Trust, which backed the research, said: "Zika and dengue come from the same family of viruses and we know they share many similarities in their genetic make-up, transmission pattern and in the immune response they trigger. These new studies suggest that prior infection with dengue doesn’t offer any protection against Zika, and may in fact predispose people to a more severe infection.
"We can’t say yet whether this interaction is playing a role in the current outbreak, but if confirmed it’s likely to have important implications for the control and global spread of Zika, and for the development of any vaccine for the virus."
He added: "There are still more questions than answers about Zika and this group of viruses including dengue. 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."
Nature Immunology June 2016
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