It depends on where you live. For most U.S. residents, Zika is unlikely to be a problem. That's because it take two factors for the virus to spread: actively infected people and the right kind of mosquitoes to spread it.
"For the average American who is not traveling, this is not a problem," Dr. Anne Schuchat of the Centers for Disease Control and Prevention says. Only Aedes aegypti mosquitoes are known for sure to spread Zika, and they're common only in the very southernmost states — south Florida, south Texas, Louisiana and Hawaii. Even there, they would have to bite someone already infected and then live long enough for the virus to build up in their salivary glands -- about 10 days.
Dengue and chikungunya, viruses closely related to Zika that are spread by the same mosquitoes, have caused only a very few, very limited outbreaks over the past few decades.
Sexual transmission is also possible, and the CDC advises people who have traveled to Zika-affected areas to use condoms or abstain from sex until they know they are in the clear. That's six months for men, because the virus can stay in semen for months.
And 80 percent of people who get Zika don't even know it. The other 20 percent may get a rash, a fever or pinkeye, but Zika hardly ever causes serious illness.
But pregnant women planning to travel should worry. "For pregnant women who to plan travel: Please take this very seriously," Schuchat said. CDC has advised pregnant women to seriously consider postponing travel to any Zika-affected regions. That's because Zika causes microcephaly, a serious birth defect in which the brain is underdeveloped.It causes other brain damage, also, in fetuses.
Hundreds of travelers have already shown up with Zika. The CDC says only cases aerious enough to merit a doctor's attention get reported, so many thousands of others have almost certainly not been reported. Zika is spreading at epidemic rates in Puerto Rico, a U.S. territory, and on other Caribbean islands. Epidemics are not likely even in the southern U.S., however.
There are maps that show the known distribution of Aedes aegypti mosquitoes and their cousins, Aedes albopictus. "Don't make the assumption that you are going to see outbreaks in that distribution of mosquitoes," says Dr. Tony Fauci, director of the National Institute of Allergies and Infectious Diseases. "That's just where the mosquitoes are." Those mosquitoes would need a pool of infected humans to bite before they could spread the virus. Right now, Zika is spreading fastest in tropical cities with dense populations and poor mosquito control. "We do think the living conditions in general in the United States — the lack of density, better air conditioning, wider use of screens — will keep us in better shape," Fauci said.
Florida has the dual risk of having the right mosquitoes and lots of travel to and from the affected areas. "We consider Florida a prime risk," said Jorge Rey, an entomologist at the University of Florida. "If you look at past histories, chikungunya and dengue have been transmitted in the Keys."
Not unless the mosquito has it. So if it was a mosquito in in a U.S. area where local transmission of Zika hasn't been seen the answer is almost certainly "no." If you are in a Zika-affected region, the answer is "maybe." Researchers do not know the attack rate — how many people get infected once Zika's in an area. They also do not know whether a single bite can transmit the virus or precisely how long the mosquito has to have been infected to spread it. And if it's not an Aedes mosquito, also called a yellow fever mosquito, it does not transmit the virus.
You should definitely take precautions against getting bitten. That means long sleeves and long pants, use of a mosquito repellent such as DEET and staying inside when possible. There's a chemical called permethrin that you can spray on your clothing and gear to repel mosquitoes. And if you were planning on starting a family right away, you might want to rethink that. Women who might be pregnant are advised to avoid affected areas. Doctors know that the earlier in pregnancy a woman gets exposed to a virus or a toxin that can cause brain damage, the worse the effects are on the fetus.
Zika can be transmitted by sex so men who could possibly have been infected, even without having symptoms, should avoid unprotected sex for six months, just to make sure they don't infect a pregnant woman.
Try not to worry. CDC says doctors should ask all pregnant women about their travel histories, and if they have symptoms of a possible Zika infection, they should get tested. Some clinics are testing all pregnant women who have been to an affected area. And women who have been to an affected area and whose fetus looks like it may have microcephaly on an ultrasound should also be tested. Then women should continue to get regular ultrasounds to watch for any signs of trouble.
For women with no symptoms, it's not clear what they could or should do. It appears Zika can cause borth defects even if a woman didn't have symptoms. "There are other viral conditions that can do that," Schuchat says. "That's why we are taking this very seriously. It's difficult for women to hear and one of the reasons we encouraged women who are pregnant to postpone travel to affected areas."
And no one knows how many women who get Zika while pregnant will have a baby with microcephaly or other birth defects.
Any traveler who has just returned from a Zika-affected country with an active infection could potentially get bitten by a mosquito and spread Zika that way — if the right mosquitoes are around. The infection lasts for a few days to a week. It's unlikely anyone could transmit the virus after a week. So if you don't live somewhere with Aedes aegypti mosquitoes, or if your neighbor hasn't just returned with an infection, the answer is "no." And the mosquitoes won't fly in from affected areas. Mosquitoes cannot fly more than a few hundred yards.
People have been trying for hundreds of years to do that. Mosquito control can work — wetlands draining and aggressive control with pesticides. But wide use of the best bug killers, like DDT, stopped when their effects on the environment outweighed the benefits. Brazil actually did eradicate the Aedes aegypti mosquitoes that spread Zika, chikungunya, yellow fever and other diseases in the 1950s, but the mosquitoes gradually returned.
"Controlling this type of mosquito is difficult," Schuchat said. "It is easy to say, 'Get rid of the mosquito.' It is a lot harder to do it."
It's very hard to kill Aedes mosquitoes because they like to breed in unexpected places, such as bottle caps filled with water, trash cans and discarded tires. Brazil is experimenting with genetically engineered mosquitoes that lay dud eggs, but it's an early experiment. It's also trying fish that eat mosquito larvae, but that approach doesn't work for urban breeding sites.
Maggie Fox is a senior writer for NBC News and TODAY, covering health policy, science, medical treatments and disease.