The World Health Organisation (WHO) has declared a global health emergency following a surge in birth defects linked to the Zika virus.

Zika has been linked to Brazil's spike in cases of microcephaly, a condition in which babies are born with abnormally small heads and underdeveloped brains.

The rapid spread of the virus has prompted governments around the world to advise pregnant women not to travel to areas where it has been detected.

The last time the global health agency declared an international emergency was during the Ebola outbreak in 2014.

WHO officials have said the Zika virus is spreading “explosively” through the Americas and could infect 3 to 4 million people in the region over the coming year.

The mosquito-borne illness has also been found in people who have traveled from the tropics to the US, Canada and Europe.

Here are some facts about the Zika virus.

How does the virus spread?

The virus is transmitted through bites from infected female Aedes mosquitoes, which also spread other tropical diseases like dengue, chikungunya and yellow fever.

Aedes mosquitoes are found in all countries in the Americas, including along the southern United States. Canada and Chile are the only countries in the region that don’t have this mosquito.

This year’s El Niño weather system is expected to fuel the outbreak by increasing the mosquito population.

Experts are looking at whether the virus can be passed on through sex, but have said more evidence is needed to confirm this as a means of transmission. Scientists have emphasized that research shows Zika is primarily transmitted through the bite of an infected mosquito.

How dangerous is it?

The biggest health threat is to unborn children during pregnancy. Around 80% of people infected never develop symptoms. And those that do typically have mild symptoms – fever, rash, conjunctivitis, muscle and joint pain and fatigue – that can last for up to a week. This makes it difficult for pregnant women to know whether they have been infected.

The connection between Zika and microcephaly is still being investigated. WHO director-general Margaret Chan, says the link was “strongly suspected but not yet scientifically proven”.

In Brazil, almost 4,000 cases of microcephaly thought to be linked to the virus have been recorded. The majority of cases appear to be in the regions where Zika is most active.

Research suggests the greatest microcephaly risk appears to be associated with infection during the first three months of pregnancy.

French Polynesia also had an outbreak of Zika virus that is believed to have affected around 11% of the population in 2013-2014. Health officials in the country documented a connection between Zika and neurological complications following the spread of the virus.

It has been associated with Guillain-Barré syndrome, a rare neurological disorder that can cause paralysis. Colombia, which has reported more than 20,000 Zika infections, including over 2,000 in pregnant women, has reported an "explosion" of Guillain-Barré syndrome cases.

Where did it start?

The virus was first discovered in Uganda in 1947 in rhesus monkeys (its name comes from the Zika forest in Uganda). It was first seen in people in 1952 in Uganda and Tanzania, according to the WHO.

Outbreaks have been recorded in Africa, the Americas, Southern Asia and Western Pacific.

Can it be contained?

There is no vaccine or medication that can prevent Zika. Eliminating mosquito breeding sites and taking precautions against bites such as using insect repellent and mosquito nets can help prevent the spread of the virus.

WHO says the priority is to protect pregnant women and their babies from harm and to control the Aedes mosquitoes that are spreading the virus.

The agency advises pregnant women to avoid travel to places where they may be exposed to Zika. Women living in areas affected by Zika are being urged to seek advice from their doctor and protect themselves against mosquito bites.