One hospital in Pernambuco - the Brazilian state with most suspected cases of Zika tied to microcephaly in newborns - is advising families to put off any pregnancy plans for the time being.
The situation is grave enough to have caused six of Brazil's states to declare a state of public health emergency.
A baby born with microcephaly has a smaller-than-normal head, caused by abnormal brain growth or because the brain stops growing. The brain fails to grow as the infant develops, and the condition often results in serious neurological and development problems and sometimes early death.
One hospital in the most affected Brazilian state - Pernambuco in the northeast of the country - is advising families to put off any pregnancy plans for the time being.
Dr. Angela Rocha, the pediatric infectologist at Oswaldo Cruz Hospital in the state capital Recife, told CNN:
"It's a very personal decision, but at this moment of uncertainty, if families can put off their pregnancy plans, that's what we're recommending."
Zika is a virus that is transmitted by the same Aedes mosquito that spreads dengue and chikungunya. Infected people have clinical symptoms similar to those illnesses and it is possible that some suspected cases of dengue could be Zika.
The most common symptoms of Zika infection are mild fever, rash, headache, joint pain and non-purulent conjunctivitis (red eye with no pus). One out of four people may not develop symptoms, but those who do experience them for 2-7 days. Severe disease requiring hospitalization is uncommon.
Surge in cases of microcephaly in newborns
Outbreaks of Zika have occurred in Africa, Southeast Asia and the Pacific Islands. Because the mosquitoes that spread Zika virus are throughout the world, it is likely that outbreaks will spread to new countries. In the Americas, the virus is now endemic in nine countries in addition to Brazil: Colombia, El Salvador, Guatemala, Mexico, Panama, Paraguay, Suriname, Honduras and Venezuela.
The Centers for Disease Control and Prevention (CDC) say Zika virus is not currently found in the US, although cases have been reported in returning travelers. Similarly, in Europe, there are no reports of the virus spreading, although there was one case was reported in the Netherlands in a traveler returning from Suriname in late November 2015.
Brazil has been investigating potential links between Zika virus infection in pregnancy and fetal microcephaly since October 2015, when the Brazilian Ministry of Health reported an unusual surge in cases of microcephaly in newborns following a Zika virus outbreak in the northeastern states.
Up to December 12, 2015, the Brazilian ministry had been notified of 2,401 suspected cases of microcephaly linked to Zika virus infection in pregnancy. Of these, 2,165 are under investigation, 134 have been confirmed and 102 have been discarded.
While the outbreak of microcephaly linked to Zika was initially concentrated in the northeastern states, many suspected cases are now arising in states further south, including Rio de Janeiro and São Paulo, fueling fears that it is spreading across the country.
Currently, there is no evidence that Zika causes microcephaly. Further investigations and studies are needed to establish whether it is Zika or other prenatal infections, genetic risk factors or exposure to chemicals or drugs that is causing the unusual surge in cases in Brazil.
To this end, the Brazilian authorities have sent task forces to investigate the cases in five states. The first task force was sent to Pernambuco in late October 2015, after the state reported 26 cases of newborn microcephaly linked to Zika. The first priority of the task forces is to obtain an accurate clinical description of the cases.
São Paulo researchers working through the holidays
In the meantime, in the city of São Paulo - one of the world's most populous cities and home to 11 million people - a network of 31 laboratories and teams has already started investigating the link between microcephaly and Zika.
The teams - amounting to 160 researchers - are planning to work through the Christmas and New Year holidays to study Zika. Many already have projects under way.
By the 22nd December, a team at the Institute of Biomedical Sciences (ICB) at the University of São Paulo had already developed cultures for growing Zika in cells - a necessary first step for experiments and for obtaining diagnoses via DNA.
By Christmas Eve, the team had infected pregnant mice with Zika, to begin studying links between the virus and microcephaly.
Within a month, they expect to have a blood test for diagnosing Zika that is cheaper and easier than a DNA test.
This will be essential for investigating the spread of Zika in São Paulo, where it is suspected that many cases diagnosed as dengue are in fact Zika infections.
Paolo Zanotto, a professor at ICB who is coordinating the Zika research network in São Paulo, told Globo news:
"The days when this virus can circulate unnoticed are already numbered."
Earlier this year, Medical News Today learned how scientists are getting closer to developing a vaccine for dengue after discovering an antibody that prevents dengue infection in mice.