The Zika virus is "spreading explosively" in the Americas and can infect upto four million people, the WHO warned today as it issued a warning to all countries, including India, who have the vector of Aedes mosquito that also causes Dengue and Chikungunya.

The Zika virus is caused by the aggressive Aedes aegypti mosquito that also causes Dengue and Chikungunya — both the viral diseases are of great public health concern to tropical countries like India.

The outbreak began in Brazil last year and has now spread to 24 countries in the Americas, causing serious birth defects and other neurological problems like microcephaly, a condition that causes babies to be born with an abnormally small head.

Dr Marcos Espinal, Director of Communicable Diseases and Health Analysis at WHO, warned that Zika "will go everywhere the mosquito is. We should assume that. We should not wait for it to spread."

Zika originated in Africa and also exists in Asia.

Meanwhile, WHO chief Margaret Chan warned that the virus "is now spreading explosively," and the global health body expected up to four million cases of the disease.

She also expressed concern over the potential of global spread of the disease, owing to the large geographical spread of the Aedes mosquito.

Bruce Aylward, Assistant Director-General, WHO, also warned that the virus could spread to other places wherever there is Aedes mosquitoes.

"What we have to assume is anywhere where they have the Aedes (mosquitoes), they could have the Zika virus and they should have the tools to be able to look for it," he said.

"So, that's part of the reason we are trying to get the information out to countries that have got the vector but may not yet have the virus - look now for the virus," he added.

Such is the level of alarm that many American countries like El Salvador, Colombia and Ecuador have urged women to postpone pregnancy till 2018.

Complicating matters further, the Olympics in 2016 is to be held in Rio, the epicenter of the virus outbreak.

The concern is also related to lack of immunity of the population that are unexposed to the Zika virus.

Additionally, there are no vaccines, specific treatments, and rapid diagnostic tests, available to combat the virus.

However, many countries that have had the outbreak of a Zika virus, like Colombia, have not yet reported cases of microcephalis and Guillain-Barre syndrome.

There are currently four categories of countries globally: one, countries who have the virus as well as the birth malformations, like Brazil, two, countries only with the virus, like Colombia, three, countries, like India, who have the vector, and four, countries like Chile and Canada who do not have the vector.

Though, Chikungunya and Dengue are much more rampant in many parts of the world than the Zika virus, the possible association between the Aedes mosquito and neurological impairments could bring the spotlight on mosquito-borne diseases globally.

"We must use the dengue dynamics as the reference point - where you have the dengue outbreaks in the previous years, where the mosquito Aedes is present, you have a risk of Zika transmission," said Dr Sylvain Aldighieri, WHO.

The Zika virus was first detected in 1947 from a monkey in the Zika forest of Uganda.