Why has it exploded so suddenly?
There may have been big outbreaks in Africa and Asia in the past that went undetected; scientists weren't paying much attention. But the current massive epidemic was an event waiting to happen. Latin America has huge numbers of
A. aegypti, also known as the yellow fever mosquito, an important vector for Zika. (The Asian tiger mosquito,
A. albopictus,
which is on the rise around the world, is believed to be a vector as well.) In addition, nobody in the Americas had immunity to the virus. Travel makes it worse.
Aedes mosquitoes don't fly more than a few hundred meters during their lives; Zika travels from city to city and country to country when infected people get on cars, buses, trains, and planes.
These combined factors meant that the virus had the ability to spread far and fast once it had arrived.
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Do we know for sure that Zika is causing a rise in birth defects?
No. There is strong circumstantial evidence that areas in Brazil hit hard by Zika have experienced a sharp increase in the number of babies born with microcephaly, a condition in which the head is much smaller than normal because the brain fails to develop properly. But it will take at least several months before the results from the first case-control studies of pregnant women infected with Zika are available. Doctors in Brazil first noticed an increase in cases of microcephaly during ultrasounds of pregnant women in June and July, a few months after the sudden rise in Zika infections. Fetal medicine expert Manoel Sarno, who works at the Federal University of Bahia, says the pattern of brain damage he is seeing now looks distinct from microcephaly caused by other infections, such as cytomegalovirus (CMV) or rubella. He and his colleagues started a study in August that is following women infected with Zika during their pregnancy; the results could come out late summer. Similar studies are underway elsewhere in Brazil and in Colombia.