The infection has been linked to thousands of babies being born with underdeveloped brains.

Some areas have declared a state of emergency, doctors have described it as “a pandemic in progress” and some are even advising women in affected countries to delay getting pregnant.

But there is much we do not know.

What are the symptoms of Zika?

Deaths are rare and only one-in-five people infected is thought to develop symptoms.

These include:

• mild fever
• conjunctivitis (red, sore eyes)
• headache
• joint pain
• a rash

A rare nervous system disorder, Guillain-Barré syndrome, that can cause temporary paralysis has been linked to the infection.

There is no vaccine or drug treatment so patients are advised to rest and drink plenty of fluids.

But the biggest concern is the impact it could have on babies developing in the womb and the surge in microcephaly.

What is microcephaly?

It is when a baby is born with an abnormally small head, as their brain has not developed properly.

The severity varies, but it can be deadly if the brain is so underdeveloped that it cannot regulate the functions vital to life.

Children that do survive face intellectual disability and development delays.

It can be caused by infections such as rubella, substance abuse during pregnancy or genetic abnormalities.

Case study: ‘It’s not the end of the world’

The WHO says there is “scientific consensus” that Zika causes microcephaly as well as Guillain-Barre syndrome.

Some babies who died had the virus in their brain and it has been detected in placenta and amniotic fluid too.

Is it safe to try for a baby?

Some governments have advised women to delay getting pregnant until more is known.

Experts now believe Zika is linked to a broader set of complications in pregnancy, including miscarriage, stillbirth, premature birth and eye problems.

The US Centres for Disease Control says Zika lingers in the blood for about a week and can be spread by sexual intercourse.

“The virus will not cause infections in a baby that is conceived after the virus is cleared from the blood,” it says.

“There is currently no evidence that Zika-virus infection poses a risk of birth defects in future pregnancies.”

The WHO advises couples practice safer sex or abstain for at least eight weeks if they are returning from Zika-affected areas. If the man in the couple planning a pregnancy develops Zika symptoms, then this period of abstinence or safe sex should be extended to six months.

Why is it a public health emergency?

The WHO is worried that Zika is spreading far and fast, with devastating consequences.

Declaring Zika as a “public health emergency of international concern” singles the disease out as a serious global threat. It puts it in the same category of importance as Ebola.

Unlike Ebola, where the focus was on boots on the ground, with Zika the attention will be on understanding the link with microcephaly.

The WHO will co-ordinate countries’ health agencies to conduct trials to determine the risk.

It will also encourage efforts to stop the mosquito that spreads the disease as well as finding a treatment or a vaccine to stop the virus.

The work will depend on money donated by countries.

Where did Zika come from?

It was first identified in monkeys in Uganda in 1947.

The first human case was detected in Nigeria in 1954 and there have been further outbreaks in Africa, South East Asia and the Pacific Islands.

Most were small and Zika has not previously been considered a major threat to human health.

But in May 2015 it was reported in Brazil and has since spread rapidly.

“Its current explosive pandemic re-emergence is, therefore, truly remarkable,” the US National Institutes of Health said.

How does it spread?

It is spread by Aedes mosquitoes. They are the same insects that spread dengue and chikungunya virus.

They are found throughout the Americas except for Canada and Chile where it is too cold for them to survive, and across Asia.

And, unlike the mosquitoes that spread malaria, they are mostly active during the day, so bed nets offer limited protection.

If they drink the blood of an infected person they can then infect subsequent people they bite.

The WHO says sexual transmission is also possible.

How long are people infectious?

The best evidence so far suggests that people can spread the virus via mosquitoes for a week after being infected.

In semen it may persist for two weeks.

Countries have advised safe sex and a ban on blood donations for a month after just visiting such countries and for longer if they developed symptoms.

What can people do?

As there is no treatment, the only option is to reduce the risk of being bitten.

Health officials advise people to:

• use insect repellents
• cover up with long-sleeved clothes
• keep windows and doors closed

The mosquitoes lay their eggs in standing water, so people are also being told to empty buckets and flower pots.

The US Centers for Disease Control has advised pregnant women not to travel to affected areas.

What is being done?

Brazilian Health Minister Marcelo Castro has said a new testing kit is being developed to identify infections quickly.

He also said more money was being put into the development of a vaccine.

Some scientists are also trialling the use of genetically modified sterile mosquitoes that appear to reduce mosquito populations by 90%.

Meanwhile, efforts are under way to kill the mosquitoes with insecticide.

Zika vaccine

US experts from the National Institutes of Health say trials of a Zika vaccine will likely start in September this year. Depending on the results, larger trials could begin at the start of 2017.

“The very, very best scenario” would be a vaccine ready for the general public by the beginning of 2018, they say.