Answers and advice about the Zika virus outbreak.

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The World Health Organization (WHO) warned Thursday that the Zika virus is “spreading explosively” in the Americas and that up to 4 million people could be infected by the end of the year.

Officials at the U.S. Centers for Disease Control and Prevention (CDC) have urged pregnant women against travel to several countries in the Caribbean and Latin America where the outbreak is growing.

The infection appears to be linked to the development of unusually small heads and brain damage in newborns. Some pregnant women who have been to these regions should be tested for the infection, WHO also says. Some answers and advice about the outbreak.

Q: What is the Zika virus?

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A: A tropical infection new to the Western Hemisphere.

The Zika virus is a mosquito-transmitted infection related to dengue, yellow fever and West Nile virus. Although it was discovered in the Zika forest in Uganda in 1947 and is common in Africa and Asia, it did not begin spreading widely in the Western Hemisphere until May, when an outbreak occurred in Brazil.

Until now, almost no one on this side of the world had been infected. Few of us have immune defenses against the virus, so it is spreading rapidly. Millions of people in tropical regions of the Americas may have had it.

Q: How is the virus spread?

A: Mosquitoes, but not every species.

Zika is spread by mosquitoes of the Aedes species, which can breed in a pool of water as small as a bottle cap and usually bite during the day. The aggressive yellow-fever mosquito, Aedes aegypti, has spread most Zika cases, but that mosquito is common in the United States only in Florida, along the Gulf Coast, and in Hawaii — although it has been found as far north as Washington, D.C., in hot weather.

The Asian tiger mosquito, Aedes albopictus, is also known to transmit the virus, but it is not clear how efficiently. That mosquito ranges as far north as New York and Chicago in summer.

Although the virus is normally spread by mosquitoes, there has been one report of possible spread through blood transfusion and one of possible spread through sex. The virus was found once in semen.

Q: How does Zika cause brain damage in infants?

A: Experts are only beginning to figure it out. Scientists do not fully understand the connection. The possibility that the Zika virus causes microcephaly — unusually small heads and damaged brains — emerged in October, when doctors in northern Brazil noticed a surge in babies with the condition.

It may be that other factors, such as simultaneous infection with other viruses, are contributing to the rise; investigators may find that Zika virus is not the main cause, although circumstantial evidence strongly suggests that it is. It is not known how common microcephaly has become in Brazil’s outbreak.

About 3 million babies are born in Brazil each year. Normally, about 150 cases of microcephaly are reported, and Brazil says it is investigating nearly 4,000 cases.

Q: What countries should pregnant women avoid?

A: About two dozen destinations mostly in the Caribbean, Central America and South America. The Pan American Health Organization believes the virus will spread in every country in the Americas except Canada and Chile. The latest updates to the CDC’s current list of countries and territories in which Zika virus is circulating can be found on the CDC website.

Q: How do I know if I’ve been infected? Is there a test?

A: It’s often a silent infection and hard to diagnose. Until recently, Zika was not considered a major threat because its symptoms are relatively mild. Only 1 of 5 people infected with the virus develop symptoms, which can include fever, rash, joint pain and red eyes. Those infected usually do not have to be hospitalized.

There is no widely available test for Zika infection. Because it is closely related to dengue and yellow fever, it may cross-react with antibody tests for those viruses. To detect Zika, a blood or tissue sample from the first week in the infection must be sent to an advanced laboratory so the virus can be detected through sophisticated molecular testing.

Q: I’m pregnant and I recently visited a country with Zika virus. What do I do?

A: Some women should get blood tests, and just about all should get ultrasound scans. On Jan. 19, the CDC issued interim guidelines for women in that situation and for their doctors. The guidelines may change.

In general, they say that pregnant women who have visited any area with Zika transmission should consult a doctor. Those who have had symptoms of infection such as fever, rash, joint pain and bloodshot eyes during their trip or within two weeks of returning should have a blood test for the virus.

That recommendation is controversial, because even women with no symptoms may have been infected — 80 percent of those who get the virus do not feel ill — and there is no evidence babies are hurt only when the mother has been visibly ill.

At the time the guidelines were issued, the CDC and state health departments did not have the laboratory capacity to test every pregnant woman who visited Latin America and the Caribbean in the past nine months, as well as every pregnant woman in Puerto Rico.

Even for women who get blood tests, the news is not reassuring. Tests for the virus only work in the first week or so after infection. Tests for antibodies can be done later, but they may yield false positives if the woman has had dengue, yellow fever or even a yellow-fever vaccine.

Under the CDC’s testing algorithm, pregnant women who have been to affected regions — whether they have symptoms or not, and whether they have negative or positive blood tests — should eventually have ultrasound scans to see if their fetuses are developing microcephaly or calcification of the skull.

Unfortunately, an ultrasound normally cannot detect microcephaly before the end of the second trimester.

Some women also should have amniocentesis to test the fluid around the fetus for Zika virus. But amniocentesis involves piercing the amniotic sac with a long needle through the abdomen; it is slightly risky for the fetus and is not recommended before 15 weeks’ gestation.

Several companies are working on rapid tests for Zika infection. The CDC also usually distributes test kits and training materials to state health departments during outbreaks, which should increase testing capacity.

Q: Does it matter when in her pregnancy a woman is infected with Zika virus?

A: Earlier in pregnancy seems to be more dangerous. The most dangerous time is thought to be during the first trimester, when some women do not realize they are pregnant.

Experts do not know how the virus enters the placenta and damages the growing brain of the fetus. Closely related viruses, including yellow fever, dengue and West Nile, do not normally do so. Viruses from other families, including rubella (German measles) and cytomegalovirus, sometimes do.

Q: Should infants be tested?

A: Microcephaly is not the only birth defect caused by the virus. Federal health officials say that newborns should be tested for infection with the Zika virus if their mothers have visited or lived in any country experiencing an outbreak and if the mothers’ tests are positive or inconclusive.

The reason, officials said in interviews, is that infection with the virus could be linked to defects in vision and hearing, among other abnormalities, even if the child does not have microcephaly.

The new guidance applies only to infants of mothers who reported symptoms of Zika virus infection while living abroad in an affected country or within two weeks of travel to such a destination.

Q: Is there a treatment?

A: No. The CDC does not recommend a particular antiviral medication for people infected with the Zika virus.

Q: Is there a vaccine? How should people protect themselves?

A: There is no vaccine against the Zika virus. Efforts to make one have just begun, and creating and testing a vaccine normally takes years and costs hundreds of millions of dollars.

Because it is impossible to completely prevent mosquito bites, the CDC has advised pregnant women to avoid going to regions where Zika is being transmitted, and has advised women thinking of becoming pregnant to consult doctors before going.

Travelers to these countries are advised to avoid or minimize mosquito bites by staying in screened or air-conditioned rooms or sleeping under mosquito nets, wearing insect repellent at all times and wearing long pants, long sleeves, shoes and hats.