The Zika virus is likely to spread across nearly all of the Americas, the World Health Organization has warned.
The infection, which causes symptoms including mild fever, conjunctivitis and headache, has already been found in 21 countries in the Caribbean, North and South America. It has been linked to thousands of babies being born with underdeveloped brains and some countries have advised women not to get pregnant.
No treatment or vaccine is available.
Brazil says the number of babies born with suspected microcephaly or abnormally small heads since October has now reached nearly 4,000.
In the worst affected area, about 1% of newborns have suspected microcephaly.
The Brazilian authorities believe the increase is caused by an outbreak of Zika virus. Just 150 babies were born with microcephaly in 2014.
The brain condition can be deadly or cause intellectual disability and developmental delays.
Colombia’s health minister has advised women there to delay pregnancy.
Brazil’s health ministry says there have been 3,893 suspected cases of microcephaly since October, when the authorities first noticed a surge, up from 3,500 in last week’s report.
The link with Zika has not been confirmed, but a small number of babies who died had the virus in their brain and no other explanation for the surge in microcephaly has been suggested.
Zika is generally mild and only causes symptoms in one in five people. It is spread by the Aedes aegypti mosquito, which also spreads dengue and chikungunya.
Officials in four Latin American and Caribbean nations have warned women to avoid pregnancy amid concerns over an illness causing severe birth defects.
Colombia, Ecuador, El Salvador and Jamaica recommended to delay pregnancies until more was known about the mosquito-borne Zika virus.
This followed an outbreak in Brazil.
Brazil said the number of babies born with suspected microcephaly – or abnormally small heads – had reached nearly 4,000 since October.
Zika’s alarming spread: CDC investigates link to paralyzing condition, adds 8 countries to travel warning
The Centers for Disease Control and Prevention, which reported this week that a dozen cases of Zika virus have been confirmed in the United States, is expanding its advisory that pregnant women should avoid travel to countries currently seeing high rates of infection.
The agency’s initial list contained 14 countries, but the CDC on Friday added eight more — in South America, the Caribbean and Polynesia — as places where the reach of the virus is growing.
The CDC now is working with authorities in Brazil to study a potential link between the mosquito-borne virus and a rare syndrome known as Guillain-Barré that can lead to paralysis. In Brazil, which is currently the epicenter of Zika, public health officials were already investigating a link between the virus and a rare birth condition called microcephaly. That country has seen nearly 3,900 suspected cases since October, with the babies involved suffering serious brain damage.
In recent days, several countries have taken drastic measures to try to combat the virus. Brazil, Colombia, El Salvador and Jamaica are all calling on women to consider delaying becoming pregnant.
So far, the known U.S. cases are of people who were traveling and likely bitten by mosquitoes while abroad — but infectious disease experts say the rapid spread of the tropical virus in the Americas may mean an outbreak here.