Zika virus, a mosquito-borne illness that has been linked to brain deformities in babies. Pregnant women have been warned not to travel to the 22 countries where the infection has been reported.
This has raised concerns about pregnant women planning to visit Rio de Janeiro for the upcoming Olympic Games in August. Zika virus, which was first discovered in Africa in the 1940s, is rapidly spreading through south America and the Caribbean.
Three British travellers picked up the disease through mosquito bites while visiting Colombia, Suriname and Guyana and are all now back in the UK. Public Health England (PHE) could not confirm whether any of the victims were pregnant women.
PHE said Zika 'does not occur naturally in the UK' and can be transmitted only from the bite of a mosquito or, in rare cases, 'through sexual transmission or by transmission from mother to foetus via the placenta'.
It does 'not spread directly from person to person', the statement said. While the symptoms of Zika itself tend to be mild – they include joint pain and a rash – scientists believe it is linked to microcephaly, or abnormally small brains, in newborns. There is currently no vaccine or drug to prevent an infection. Only a handful of Zika cases had ever been documented before 2013.
But scientists began sounding the alarm after multiple outbreaks were discovered in Pacific islands and south-east Asia. It is thought the Zika virus - which was at first thought to be relatively innocuous - may have arrived in Brazil during the 2014 World Cup by visitors from French Polynesia, where an outbreak had just occurred.
Scientists estimate as many as 1.5 million people could now be infected in Brazil. Brazil said the number of babies born with suspected microcephaly - the medical term for abnormally small heads - had reached nearly 4,000 since October.
The number of reported deaths of deformed babies rose to 49, ministry officials said at a news conference earlier this week. But so far, health authorities have only confirmed six cases of microcephaly where the infant was infected with the mosquito-born Zika virus.
Health experts in Brazil have warned next month's Carnival celebrations could aggravate the spread of the virus. Few visitors are likely to wear protective clothing on the beach or to Carnival street parties, making them vulnerable to insect bites.
To try and combat the disease, health authorities have been conducting door-to-door searches of residences to clear them of possible mosquito breeding sites, such as standing pools of water.
Elsewhere, Brazilians are being offered cash incentives or being threatened with fines if they fail to ensure their properties are mosquito free. Colombia has the second highest infection rate, with more than 13,500 people infected with the virus and the disease could hit as many as 700,000, its health minister said.
The country's health minister, Alejandro Gaviria, urged women to delay pregnancies for up to eight months. He said: 'We are doing this because I believe it's a good way to communicate the risk, to tell people that there could be serious consequences.'
Similar warnings were issued in Ecuador, El Salvador and Jamaica.
However, women's rights campaigners criticised the recommendations, saying women in the region often had little choice about becoming pregnant.
'It's incredibly naive for a government to ask women to postpone getting pregnant in a context such as Colombia, where more than 50% of pregnancies are unplanned and across the region where sexual violence is prevalent,' said Monica Roa, a member of Women's Link Worldwide group.
The Zika virus is transmitted by the Aedes aegypti mosquito (pictured), which is also known to carry yellow fever. Zika virus is spread to people via mosquito bites.
The most common symptoms of Zika virus disease are fever, rash, joint pain and conjunctivtis. Infected patients are typically ill for a few days to a week. While the illness is generally mild, some experts in Brazil have suggested a possible link between the virus in pregnant women and subsequent birth defects.
The CDC said recently it is aware of reports of increased numbers of babies born with microcephaly, or smaller than expected head size, in Brazil. The Ministry of Health in Brazil is concerned about a possible association between the Zika virus and increased numbers of babies born with microcephaly. There is no vaccine to prevent, or medicine to treat Zika virus.
Travellers can protect themselves, by taking steps to prevent mosquito bites.
22 COUNTRIES THAT ARE AFFECTED
The US Centers for Disease Control and Prevention issued initial travel warnings to pregnant women last week, adding eight more places to the list on Friday.
The warnings now extend to:
Central and South America: Bolivia, Ecuador, Guyana, Brazil, Colombia, El Salvador, French Guiana, Guatemala, Honduras, Mexico, Panama, Paraguay, Suriname, Venezuela
Caribbean: Barbados, Saint Martin, Haiti, Martinique, Puerto Rico, Guadeloupe
Africa: Cape Verde