Women account for 63.6 percent of the cases, of whom at least 2,000 are pregnant.
What is the Zika virus?
Zika virus is a mosquito-transmitted infection related to dengue, yellow fever and West Nile virus.
Symptoms are mild and include fever, rash, joint pain and conjunctivitis (red eyes).
Just one in five people infected becomes ill. Hospitalisation is uncommon and deaths are rare.
How does it spread?
Zika is primarily spread through the bite of an infected Aedes species of mosquito.
Pregnant women can also pass the virus to their babies during pregnancy, but how and when this happens is unclear.
Those infected can pass the virus on through a mosquito bite for about seven days after infection.
There are no reports of transmission through breastfeeding, but in a few cases the virus has been reported to have been passed on through blood transfusion.
Can it be caught through sexual contact?
Sexual transmission of Zika is considered rare although health officials in the US state of Texas have reported a case in which they say Zika is likely to have been contracted through sex.
The individual infected had not been in Zika-affected areas, but a sexual partner had and suffered Zika-like symptoms.
Health officials are now urging men to use condoms after travelling to areas which have the Zika virus and to avoid having unprotected sex for six months if they have had symptoms.
They say pregnant women should avoid contact with semen from men exposed to the virus.
Where is Zika?
The latest outbreak is in 25 countries in the Carribean, Central America and South America.
Past outbreaks have been in Gabon, Egypt, Nigeria, Senegal, Sierra Leone, Côte d’Ivoire, the Central African Republic, Cambodia, Micronesia, Malaysia, Pakistan, India, Thailand, Philippines, and Indonesia.
How did Zika get to Brazil?
Nothing has been proven, but Zika may have been brought to Brazil by participants of the Va’a World Sprint Canoeing World Championships, held in August 2014.
Athletes came from French Polynesia, New Caledonia, the Cook Islands and Easter Island.
It has also been suggested that Zika was brought by Asian tourists attending the 2014 FIFA World Cup, raising concerns that the Olympics, starting in Rio de Janeiro on August 5, will result in further spread of the virus.
How is Zika diagnosed?
A blood or tissue sample from the first week of the infection must be sent to an advanced laboratory.
The virus can be detected through sophisticated molecular testing that seeks out the active virus, which lasts in the body for about a week.
Research is being done to develop a rapid test which could look for antibodies after a patient has recovered from the virus, making it possible to test for immunity.
How is Zika treated?
There is no vaccine or specific medicine currently available and treatment is normally focused on relieving the symptoms.
Can a vaccine be developed?
A vaccine is possible but development, testing and trials for human vaccines normally take years and cost hundreds of millions of dollars.
A number of companies, including Inovio, Hawaii Biotech, GSK and Sanofi are developing or considering working on a vaccine.
Indian company Bharat Biotech is reported to have applied for patents for two possible Zika vaccines last year, but these must still be tested for safety and efficacity.
Last year Sanofi received approval for the world’s first dengue vaccine, which is closely related to Zika.
In response to the current outbreak it has launched a v accine research and development project to target Zika.
Until now it has been cautious about whether the dengue vaccine could be adapted.
“There are too many unknowns about Zika to reliably judge the ability to research and develop a vaccine effectively at this time,” it said in a statement.
What is microcephaly and how is it linked?
Microcephaly is a birth defect characterised by incomplete brain development and an unusually small head.
It is a life-long condition with no cure or standard of treatment, and is linked with conditions such as seizures, developmental delays and intellectual problems.
Microcephaly is normally uncommon. In the US, there are typically between two and 12 cases per 10,000 newborns.
The condition is being reported in the worst Zika-affected areas in Brazil at dramatically higher rates: 100 cases for every 10,000, or one percent of births.
Research from Brazil has suggested there is a correlation between the location and timing of this Zika outbreak and the increase in cases of microcephaly, but there is currently no proof that the virus causes the birth defect.
On January 27, the Minsitry of Health in Brazil reported that out of 4180 suspected cases of microcephaly, only 270 had been confirmed, and of these just six were linked to Zika.
This has given rise to the view that the number of cases and the scale of the problem has been misrepresented.
On the February 1, the World Health Organization declared a global health emergency in response to the growing number of birth defects and other neurological disorders in Brazil and their possible link to the Zika virus.
The WHO’s Director-General Dr Margaret Chan says a causal link is “strongly suspected”.
Why was the potential link between Zika and birth defects not previously observed?
In areas where Zika has been active for decades, such as Central Africa and Asia, most people are infected early in life, so the risk of infection during pregnancy is small.
The population in the Americas has not had exposure to the virus until now.
This means more women are now being exposed to the virus for the first time during pregnancy, possibly increasing the risk of birth defects.
This is why public health officials are telling women to avoid getting pregnant, at least until they have had the virus, or Zika has been brought under control.
Can GM mosquitos stop the virus?
British biotech company Oxitec has produced genetically modified Aedes aegypti mosquitoes by introduing two genes into its DNA.
One of them makes its eggs glow under UV light, helping with identification.
The other causes the mosquito’s offspring to die.
Oxitec says that by releasing the GM mosquitos into infected areas, populations of Aedes aegypti can be reduced by more than 80 percent, thereby reducing transmission of Zika.
Oxitec says there is no way the mosquitos’ modified DNA can transfer into humans or other mammals and insects, but public fears over genetic modification mean the technique is controversial.