With the sting of a mosquito bite and a fever, many pregnant women may not know that they caught the Zika virus - until it strikes their unborn child.
But thousands of cases of birth defects linked to the disease over recent months are now sowing alarm in the US and in Latin America.
Babies have been born with abnormally smaller heads, a condition doctors call microcephaly, which often causes brain damage.
The scare has struck hardest in Brazil, which hosts the summer Olympic Games in August.
It is one of 14 Latin American and Caribbean countries that the US has warned pregnant women not to visit because of the Zika risk.
“I am very afraid,” said Jacinta Silva Goes, a 39-year-old cleaning lady in Sao Paulo who is expecting her third child.
“For the moment I am not using mosquito repellent because the doctor has not told me to. He has not spoken to me about Zika,” she said.
Zika used to be thought just a poor relation of dengue, another mosquito-borne virus.
The World Health Organisation (WHO) says it is transmitted by the same mosquito that spreads dengue, chikungunya and yellow fever.
But the WHO this week noted a surge in cases of microcephaly in Brazil, the country most affected by the current epidemic.
It stressed that “the causal link between Zika infection and microcephaly has not yet been proven,” though the rise in cases is striking.
In the past year there were 3,530 cases of microcephaly reported, compared to a yearly average of 163 over the four preceding years.
Scientists from Brazil’s Fiocruz Institute and Parana Catholic University published a study of a case that showed the virus had passed from an infected woman to her baby through the placenta.
Goes says she feels uninformed about how to protect herself against Zika.
“I can’t decide by myself to take anything until the doctor tells me to because it could be dangerous.”
But across the vast region of Latin America, citizens and officials have taken a variety of measures in response to the scare.
The price of mosquito repellent has risen in Brazil.
In the Peruvian capital Lima, authorities disinfected a big cemetery for fear that Zika-bearing mosquitos were breeding in the flower pots.
In Colombia the government simply advised couples to avoid pregnancy until at least June. El Salvador, which has reported more than 5,500 cases, said avoid getting pregnant until 2017.
Originating in East Africa, Zika landed in Latin America last year and has spread across virtually the whole region via Aedes aegypti, a species of mosquito, says the Panamerican Health Organisation.
It says Zika is present in 18 countries in the region. There is no overall figure for the number of cases detected.
With its high number of microcephaly cases, Brazil is the country by far the worst affected. It is followed by Colombia, which has reported 13,531 cases and is predicting up to 700,000 this year.
Zika has also been reported in the US: three cases were detected in Florida in people who had recently traveled to Latin America.
And the federal Centers for Disease Control and Prevention (CDC) said that a newborn in Hawaii, was found to have brain damage linked to Zika.
The baby was born to a woman who had been living in Brazil early in her pregnancy.
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