Here are five important facts about the virus:
It rarely causes serious symptoms
Zika has flown under most people's radar until now because it usually causes such mild symptoms that three-quarters of infected people don't even notice it. Symptoms include a fever, sometimes a rash, conjunctivitis and headache. It may occasionally cause a rare neurological condition called Guillain-Barre syndrome that's sometimes seen after an infection.
It's spread by mosquitoes
Zika is spread by the Aedes genus of mosquitoes. That includes the Aedes aegypti mosquito, which also spreads dengue virus, yellow fever virus and Chikungunya. It can also be spread by Aedes albopictus mosquitoes, commonly known as the Asian tiger mosquito. Aedes aegypti are restricted to tropical and subtropical regions but that's a broad area - as far north as Georgia and South Carolina in the U.S. The Asian tiger mosquitoes have an even bigger range and are found across the entire southeastern U.S., into Missouri and Oklahoma and as far north as New York and into temperate areas of South America.
People infected with Zika virus don't infect one another. But, just as with malaria, people are the source for spreading the virus. A female mosquito bites an infected person and then can carry the virus to the next person she bites, so when people travel,they can bring the virus with them. The virus can take hold if enough people become infected for it to become endemic -- a word meaning it's in a region permanently.
It's likely to spread fast and far
Anyone curious about how far Zika will spread need only look at Chikungunya. Chikungunya had never been seen outside Africa and parts of Southeast Asia until about 2005. It spread quickly after that and once it hit the western hemisphere it took just months to spread across the Caribbean to central and then South America. Experts fully expect Chikungunya to establish itself in the southern U.S. at some point. One piece of good news for the U.S. is that Zika is not likely to spread as fast as West Nile virus did. West Nile virus, which has reached all of continental North America since it was first seen in New York in 1999, lives in birds and infects many more different species of mosquitoes, giving it more places to lurk.
As with all mosquito-borne infections, the best way to control it is to control mosquitoes.
It's not certain Zika causes birth defects
Zika's suspected of causing a severe birth defect called microcephaly, which causes underdevelopment of the head and brain. Babies with microcephaly often miscarry before they are born, or they die at birth. Those who survive are usually very disabled. Other viruses are known to cause microcephaly if a pregnant woman is infected - rubella is the most notorious. But Zika belongs to a family of viruses not known to cause birth defects and birth defects had not been noticed in countries that had Zika previously. It's also not entirely clear to what degree cases of microcephaly have increased in Brazil. There are some smoking guns - some babies born with microcephaly or their mothers have evidence of the virus. But it's hard to test for Zika and there's no quick, on-the-spot test, making research difficult.
There's no treatment and no vaccine
As with so many viruses, there is no specific treatment for Zika. Antibiotics can treat a range of bacterial infections but the same is not true of antiviral drugs. There had not been any market for a drug to treat Zika since it did not seem to cause severe health problems. There's also no vaccine, for the same reason. But experts say it should be easy to make a vaccine against Zika.