The World Health Organisation has warned that the Zika virus is "spreading explosively" in the Americas and that as many as four million people could be infected by the end of the year.
In the US, officials at the Centres for Disease Control and Prevention 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, the agency also says. Here are some answers and advice about the outbreak.
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 last 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.
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 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 on one occasion in semen.
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 even find that Zika virus is not the main cause, although right now circumstantial evidence strongly suggests that it is.
It is not known how common microcephaly has become in Brazil's outbreak. About three million babies are born in Brazil each year. Normally, about 150 cases of microcephaly are reported, and Brazil says it is investigating nearly 4000 cases.
About two dozen destinations mostly in the Caribbean, Central America and South America.
The Pan American Health Organisation believes that the virus will spread locally 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 here.
Countries and territories with active Zika virus transmission:
Barbados
Bolivia
Brazil
Cape Verde
Colombia
Dominican Republic
Ecuador
El Salvador
French Guiana
Guadeloupe
Guatemala
Guyana
Haiti
Honduras
Martinique
Mexico
Panama
Paraguay
Puerto Rico
Saint Martin
Samoa
Suriname
US Virgin Islands
Venezuela
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 one of five 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 hospitalised.
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.
Some women should get blood tests, and just about all should get ultrasound scans.
On January 19, the CDC issued interim guidelines for women in that situation and for their doctors. The guidelines are complex and 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 like 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 per cent of those who get the virus have do not feel ill - and there is no evidence that babies are hurt only when the mother has been visibly ill. But at the time the guidelines were issued, the CDC and state health departments simply did not have the laboratory capacity to test every pregnant woman who visited Latin America and the Caribbean in the last nine months, as well as every pregnant woman in Puerto Rico.
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 realise 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.
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' own 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 suffer microcephaly. The other defects may require further assessments and testing.
The new guidance applies only to infants of mothers who reported symptoms of Zika virus infection - a rash, joint pain, red eyes or fever - while living abroad in an affected country or within two weeks of travel to such a destination.
No. The CDC does not recommend a particular antiviral medication for people infected with the Zika virus. The symptoms are mild when they appear at all and usually require only rest, nourishment and other supportive care.
Protection is difficult in mosquito-infested regions.
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.
Travellers to these countries are advised to avoid or minimise 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.
What can travelers do to prevent Zika?
The CDC website states, travellers can protect themselves by preventing mosquito bites by doing the following things:
Until now, the virus never struck such a large population without immunity.
Microcephaly is rare, and it has many other causes, including infection of the fetus with rubella (German measles), cytomegalovirus or toxoplasmosis (cat-litter disease); poisoning of the fetus by alcohol, mercury or radiation; or severe maternal malnutrition and diabetes. It is also caused by several gene mutations, including Down syndrome.
Until recently, health officials paid little attention to Zika virus. It circulated in the same regions as dengue and chikungunya, and compared to those two painful infections - nicknamed "break-bone fever" and "bending-up fever" - Zika was usually mild.
The virus is thought to have reached Asia from Africa at least 50 years ago. While it may have caused spikes in microcephaly as it first spread, there was no testing to pin down which of many possible causes was to blame.
In 2007, a Southeast Asian strain of the Zika virus began leap-frogging the South Pacific, sparking rapid outbreaks on islands where no one had immunity to it. Because island populations are small, rare side effects did not occur often enough to be noticed. But in 2013, during an outbreak in French Polynesia, which has 270,000 residents, doctors confirmed 42 cases of Guillain-Barr-syndrome, which can cause paralysis. That was about eight times the normal number and the first hint that Zika virus can attack the nervous system, which includes the brain.
Zika was first confirmed in Brazil, a country of 200 million, last May, and it spread like wildfire. The first alarms about microcephaly were raised only in October, when doctors in the northeastern state of Pernambuco reported a surge in babies born with it. Pernambuco has nine million people and 129,000 annual births. In a typical year, nine are microcephalic infants.
By November 2015, when Brazil declared a health emergency, Pernambuco had had 646 such births.
The New York Times with Traveller.com.au
