The mosquito-borne Zika virus, which has been linked to brain damage in thousands of babies in Brazil, is likely to spread to all countries in the Americas except for Canada and Chile, the World Health Organization said on Monday.
Zika transmission has not yet been reported in the continental United States, although a woman who fell ill with the virus in Brazil later gave birth to a brain-damaged baby in Hawaii.
Brazil's Health Ministry said in November that Zika was linked to a fetal deformation known as microcephaly, in which infants are born with smaller-than-usual brains.
The virus was first found in a monkey in the Zika forest near Lake Victoria, Uganda, in 1947, and has historically occurred in parts of Africa, Southeast Asia and the Pacific Islands. But there is little scientific data on it and it is unclear why it might be causing microcephaly in Brazil.
Laura Rodrigues of the London School of Hygiene and Tropical Medicine said it was possible the disease could be evolving.
If the epidemic was still going on in August, when Brazil is due to host the Olympic Games in Rio de Janeiro, then pregnant women should either stay away or be obsessive about covering up against mosquito bites, she said.
The WHO advised pregnant women planning to travel to areas where Zika is circulating to consult a healthcare provider before traveling and on return.
The clinical symptoms of Zika are usually mild and often similar to dengue, a fever which is transmitted by the same Aedes aegypti mosquito, leading to fears that Zika will spread into all parts of the world where dengue is commonplace.
More than one-third of the world’s population lives in areas at risk of dengue infection, in a band stretching through Africa, India, Southeast Asia and Latin America.
Zika's rapid spread, to 21 countries and territories in the Americas since May 2015, is due to the prevalence of Aedes aegypti and a lack of immunity among the population, the WHO said in a statement.
Like rubella, which also causes mild symptoms but can lead to birth defects, health experts believe a vaccine is needed to protect girls before they reach childbearing age.
Evidence about other transmission routes, apart from mosquito bites, is limited.
"Zika has been isolated in human semen, and one case of possible person-to-person sexual transmission has been described. However, more evidence is needed to confirm whether sexual contact is a means of Zika transmission," the WHO said.
While a causal link between Zika and microcephaly has not yet been definitively proven, WHO Director-General Margaret Chan said the circumstantial evidence was "suggestive and extremely worrisome".
In addition to finding a vaccine and potential drugs to fight Zika, some scientists are also planning to take the fight to the mosquitoes that carry the disease.
Oxitec, the UK subsidiary of U.S. synthetic biology company Intrexon, hopes to deploy a self-limiting genetically modified strain of insects to compete with normal Aedes aegypti.
Oxitec says its proprietary OX513A mosquito succeeded in reducing wild larvae of the Aedes mosquito by 82 percent in an area of Brazil where 25 million of the transgenic insects were released between April and November. Authorities reported a big drop in dengue cases in the area.