The World Health Organisation (WHO) has warned that the mosquito-borne Zika virus has transitioned from a mild threat to one of “alarming proportions”, as researchers examine whether it had spread to a type of mosquito found throughout the world, including Australia.

WHO director-general Margaret Chan said on Friday (AEDT) the virus was “now spreading explosively”.

“The level of alarm is extremely high,” Ms Chan said during a briefing meeting with board members in Switzerland, WHO reported.

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“The level of concern is high, as is the level of uncertainty. Questions abound. We need to get some answers quickly.”

The virus may have spread to the more common Culex, like this one. Photo: Getty

The virus, for which there is no vaccine, is already in an estimated 23 countries in the Americas, Pacific Islands and Asia. It could spread to between three and four million people, according to WHO infectious disease expert Marcos Espinal.

A team of researchers at Brazil’s Oswaldo Cruz Foundation are investigating the possibility that Zika had spread from the less common Aedes type of mosquito to the common Culex, which would heighten the threat level even further. As an Australian example, the Aedes is found only in Queensland, whereas varieties of Culex mosquitos live across the nation, according to the University of Sydney.

Brazil health officials linked the virus to an abnormally high number — almost 4000 — cases of microcephaly, a birth defect that shrinks the brain and skull.

The WHO’s Ms Chan said the causal relationship between the virus and these birth defects was “strongly suspected” and that the health body was acting on these suspicions rather than waiting for evidence beyond doubt because of the probable “heart-breaking burden on families and communities”.

The history of the virus

The zirus started in Ugandan monkeys, WHO says. Photo: Getty

WHO’s director-general explained during the briefing that Zika was first discovered in a Ugandan monkey in 1947 and “slumbered” for years before beginning its ‘explosive’ spread in 2007.

“Its historical home has been in a narrow equatorial belt stretching across Africa and into equatorial Asia,” Ms Chan said.

“For decades, the disease, transmitted by the Aedes genus of mosquito, slumbered, affecting mainly monkeys. In humans, Zika occasionally caused a mild disease of low concern.

“In 2007, Zika expanded its geographical range to cause the first documented outbreak in the Pacific islands, in the Federated States of Micronesia. From 2013-2014, 4 additional Pacific island nations documented large Zika outbreaks.

“The situation today is dramatically different. Last year, the virus was detected in the Americas, where it is now spreading explosively. As of today, cases have been reported in 23 countries and territories in the region.”

Travel risk

The virus is “strongly suspected” of causing microcephaly. Photo: Getty

Various official bodies, including the Australian Olympic Committee and the Australian Department of Foreign Affairs, have issued muted warnings to those considering travel to the Americas.

The 2016 Olympic Games will be held in Rio de Janeiro in August.

Australia’s olympic body warned women who might be pregnant at the time of the Games to reconsider attending.

While Brazil is the worst affected, Zika is thought to also be spreading in: Puerto Rico, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Barbados, Bolivia, Ecuador, Guadeloupe, Saint Martin, Guyana, Cape Verde and Samoa.