Experts say the frightening mosquito-borne Zika virus - thought to be responsible for horrific birth defects in Brazil and elsewhere - could strike Australia's tropical north.

The World Health Organisation has warned that Zika is "spreading explosively" across the globe and that four million people could be infected with it this year.

Scientists say the virus, which appears to be related to dengue fever and yellow fever could be spread in Australia by two species of mosquito.

Most concern and public fear about Zika centres around a possible link between infection with the virus during pregnancy and distressing birth defects including microcephaly.

The federal government's Smart Traveller website has warned pregnant women or those planning to conceive to reconsider plans to visit any of the affected countries.

The spread of the disease in Australia is possible, experts say. Transmission could happen anywhere there are the mosquitoes which can pass the virus from one human to another.

Aedes aegypti  - the "Yellow Fever Mosquito" - is the main transmitter of Zika globally. It is found in far north Queensland where it has been blamed for the spread of dengue fever.

Dr Cameron Webb, a mosquito-borne virus expert, says A. aegypti would play the most important role in the transmission of Zika if it does arrive here this year.

But another mosquito could have a far bigger impact.

Dr Webb, of Sydney University, says A. aegypti is unlikely to become established in southern cities of Australia even with a warming climate, but another insect, the Asian Tiger Mosquito, could survive in cooler climates.

This species is not in Australia at the moment, but it is found in the Torres Strait, and with Zika already found in Indonesia, the threat is growing closer.

Is Zika in Australia already?

Scientists in Recife, Brazil, collect mosquitoes for study. Photo: AP

According to a spokeswoman for the federal health department, "there have never been any locally acquired cases of Zika virus infection reported in Australia".

Six people tested positive for the virus in Australia last year, and Fairfax Media understands the first imported Zika case detected in Australia was in 2012. There was a single case in 2013 and 12 cases in 2014.

All of these people are believed to have been travellers who had returned from infected areas such as Brazil or the South Pacific.

For the virus to spread here, an A. aegypti mosquito would have to bite an infected traveller shortly after they returned from the country where they'd contracted the virus.

The infected mozzie would then go on to bite a local.

The chances are small but it is not impossible.

Associate Professor Nigel Beebe from the University of Queensland said given that Zika had been in French Polynesia and the Western Pacific in previous years, it was "probably more due to luck" that Australia had been spared local transmission of the virus so far.

How could Zika's spread in Australia be stopped?

Workers fumigate the Sambadrome ahead of Carnival celebrations in Rio de Janiero. Similar measures would be needed if the virus took hold in Australia. Photo: Bloomberg

Virus expert Professor Trudie Lang, of the University of Oxford, said the virus was not like Ebola in that there was no evidence for human-to-human transmission.

"Zika is not contagious," she said. "However, this is an important emerging diseases outbreak situation and we really must apply the lessons that we learnt from Ebola, because Zika could be a major public health issue [in Brazil and wider regions infected with Zika].

"There are many unknowns and so research is urgently needed to understand what is happening and how to prevent further cases."

A. aegypti mosquitoes breed in tiny water pools and usually bite during the day (especially at dawn or at dusk).

If there was an Australian outbreak, Dr Webb said local health authorities would try to kill the mosquitoes and their eggs while infected people would be isolated.

Associate Professor Beebe said there were traps designed to catch Asian Tiger Mosquitoes and other species within 400 metres of every Australian port.

If Zika does arrive this year, he said Australia's health system was well-equipped to diagnose it and contain it quickly.

"If you can diagnose it early, you can go in and suppress it," he said. 

Is there a vaccine?

There is no cure or vaccine yet. According to WHO, symptoms can be treated with standard pain and fever medicines, rest and water.

There is no cure or vaccine for Zika yet - but scientists are working on it. Photo: Jonathan Drake

How will I know I've got it?

There is no widely available test. A laboratory can confirm the diagnosis by blood tests.

Most diagnoses of  Zika virus are based on people's symptoms and travel history.

Symptoms are usually mild and appear a few days after a person is bitten by the infected mosquito.

In many cases, infected people present with slight fever and rash. Some get conjunctivitis, muscle and joint pain. Tiredness is another common symptom.

The symptoms usually last less than a week.

Should pregnant women be worried about Zika?​

Estafany Perreira holds her five-month-old nephew David Henrique Ferreira, who has microcephaly. Photo: Getty Images

Yes. Pretty much all the risk is for pregnant women travelling to areas in the world where there are Zika cases emerging.

Although much is unknown, there is a possibility that the Zika virus causes microcephaly – a condition in which babies are born with an abnormally small head.

Doctors in northern Brazil noticed a surge in babies with the condition in October and are worried there is a connection.

What about the Olympics?

Hundreds of Australian fans visited Brazil to cheer on the Socceroos during the 2014 World Cup.

With about 500,000 people expected to visit Brazil for the Olympics later this year, researchers are scrambling to figure out how much of a risk the Games might pose in spreading the Zika virus around the world.

Brazilian researchers believe that Zika came to their country during the 2014 World Cup when hundreds of thousands of visitors visited Brazil.

On Friday the International Olympic Committee president Thomas Bach said guidelines would be sent out "today or tomorrow at the latest" as Rio prepares to host the world's biggest sporting event and the first Olympics on South American soil.

Mr Bach told reporters in Athens they'll do everything to ensure the health of athletes and all visitors.

Hasn't the virus been in Africa and Asia for decades?

The virus was discovered in the Zika forest in Uganda in 1947 and is common in Africa and Asia but it did not begin spreading widely in the Western Hemisphere until last May, when an outbreak occurred in Brazil. Until now, the virus never struck such a large population without immunity.

Until recently, health officials paid little attention to Zika virus. It circulated in the same regions as dengue and chikungunya, and compared to those two painful infections - nicknamed "break-bone fever" and "bending-up fever" - Zika was usually mild.

In 2007, a Southeast Asian strain of the Zika virus began leap-frogging the South Pacific, sparking rapid outbreaks on islands where no one had immunity to it. Because island populations are small, rare side effects did not occur often enough to be noticed. But in 2013, during an outbreak in French Polynesia, which has 270,000 residents, doctors confirmed 42 cases of Guillain-Barre syndrome, which can cause paralysis. That was about eight times the normal number and the first hint that Zika virus can attack the nervous system, which includes the brain.

Zika was first confirmed in Brazil, a country of 200 million, last May, and it spread like wildfire. The first alarms about microcephaly were raised only in October, when doctors in the northeastern state of Pernambuco reported a surge in babies born with it. Pernambuco has nine million people and 129,000 annual births. In a typical year, nine are microcephalic infants.

By November 2015, when Brazil declared a health emergency, Pernambuco had had 646 such births.

What can I do to protect myself?

Fly spray is a good idea - but check the label first.

WHO is not recommending any travel or trade restrictions related to Zika virus. However, the Australian government is advising women who are pregnant (in any trimester) or who plan to become pregnant to consider postponing travel to any area where Zika virus transmission is ongoing.

Areas where Zika virus transmission is ongoing are difficult to determine and likely to change over time. However, warnings now exists for countries including: 

• Barbados
• Bolivia 
• Brazil 
• Cape Verde 
• Colombia 
• Ecuador
• El Salvador 
• French Guiana 
• Guadelope 
• Guatemala 
• Guyana 
• Haiti 
• Honduras
• Martinique
• Mexico
• Panama 
• Paraguay 
• Puerto Rico 
• Saint Martin
• Samoa 
• Suriname 
• Venezuela.

As the best protection from Zika virus is prevention, travellers should stay informed about Zika virus and other mosquito-borne diseases. 

Don't get bitten

All travellers are advised to take the following mosquito bite prevention measures:

• Apply insect repellent and wear light-coloured clothes to prevent bites.
• Use screens or close doors and windows in houses, and use mosquito nets for sleeping.
• Don't leave containers that hold at hold water (such as buckets, pots) lying about.
• Cover exposed skin by wearing long-sleeved shirts and long pants.
• Use permethrin-treated clothing and gear (such as boots, pants, socks, and tents). You can buy pre-treated clothing and gear or treat them yourself.

With Washington Post, New York TImes