The world's latest health scare is Zika: a tropical, mosquito-borne virus that holds little danger for most people, but could completely derail the lives of some.
Transmitted by sexual contact
Most often, the virus which is sweeping Latin America causes only mild symptoms with many people never even realising they have it.
But health officials suspect it may be behind a surge in cases of microcephaly, a condition which causes babies to develop abnormally small brains and skulls leading to permanent disability or death.
The outbreak has seen some countries urging couples not to get pregnant, while the US Centres for Disease Control (CDC) has warned pregnant women to avoid travelling to 22 affected nations.
On Tuesday, US health authorities confirmed the virus had been transmitted by sexual contact by someone infected in Venezuela who came home to a partner in Texas.
Here's what we know about the virus.
What is Zika?
The virus gets its name comes from a forest in Uganda where it was first discovered in infected rhesus monkeys in 1947. Within years, the virus had spread to humans in Uganda and Tanzania, according to the World Health Organisation (WHO).
Like dengue fever and chikungunya, two similar diseases, Zika is transmitted by mosquito species found in tropical and sub-tropical regions: Aedes aegypti and Aedes albopictus, or tiger mosquitoes.
How do I know if I have it?
In 70 to 80 percent of cases, the disease goes unnoticed. The symptoms resemble those of a mild case of the flu headache, muscle and joint pain, and mild fever plus a rash.
Symptoms last about two to seven days.
Why are experts worried?
The virus is suspected of causing two serious complications, though it has not been scientifically linked to either: microcephaly in babies, and Guillain-Barre syndrome.
A surge in microcephaly and other brain deformities in newborns has been reported in areas hit by the current Zika outbreak, particularly in Brazil.
Microcephaly cases in the South American country surged from 163 live births on average per year, to more than 4,000 in the current outbreak. Forty-nine babies have died.
Many foetuses die in the womb.
Guillain-Barre is a disorder in which the immune system attacks the nervous system, causing muscle weakness and sometimes paralysis.
Most patients recover, but the syndrome can sometimes be deadly. Cases believed to be linked to Zika have been reported in Brazil and French Polynesia.
What to do?
There is no vaccine for Zika, and no treatment – patients can take pain-killers and other medication to relieve the symptoms.
The virus is transmitted mainly through mosquito bites, so killing mosquitoes and avoiding contact with them is key. Health officials recommend covering up, using insect repellent and keeping windows closed or screened.
Authorities have responded to the outbreak by fumigating and getting rid of standing water where mosquitoes breed.
Watch: Sexually transmitted Zika virus reported in Texas
And officials in Brazil, Colombia, Ecuador, El Salvador and Jamaica have advised couples not to fall pregnant at this time.
Brazil has announced a crackdown on mosquito breeding grounds ahead of the Olympics, which will bring hundreds of thousands of travellers from around the world to Rio de Janeiro in August.
Where is Zika now?
The virus was first reported in Africa, Asia and the Pacific before leaping to the Americas last year.
The Aedes aegypti mosquito's habitat, mainly in tropical zones, stretches as far as the southern United States and into southern Europe.
On Monday, the WHO said Zika constitutes a "public health emergency of international concern".
It is now present in some 20 Latin American and Caribbean countries. Brazil has been the hardest hit, with almost 1.5 million infections.
Travellers have brought it back to the US states of Florida, Hawaii, New York, and Texas.
In Hawaii, a woman who had contracted Zika in Brazil gave birth to a baby with microcephaly.
Cape Verde, Indonesia, Thailand and islands in the French Caribbean have also reported Zika infections.