The WHO anticipates the Zika virus will likely spread to all but two countries in South, Central and North America.

The mosquito-borne disease has raged in South America and other regions for several months.

Twenty one countries and territories of the Americas have reported cases of the virus since Brazil reported the first cases of local transmission in May 2015, WHO’s regional office for the Americas said in a statement.

“Aedes mosquitoes—the main vector for Zika transmission—are present in all the region’s countries except Canada and continental Chile,” the statement said.

The Centers for Disease Control and Prevention urged pregnant women to postpone travel to Bolivia, Brazil, Cape Verde, Colombia, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Saint Martin, Suriname, Samoa, Venezuela and Puerto Rico. The CDC also recommended women who have recently traveled to these places during their pregnancy be screened and monitored for the virus.

That’s because the virus has been linked to an uptick in babies born with a neurological condition called microcephaly which can cause abnormally small heads and serious, sometimes deadly, developmental delays.

The WHO attributed the virus’s rapid spread to the fact that people in the Americas lack immunity because they haven’t been exposed to it before.

No prevention, treatment

There is no prevention or treatment for the disease. Travelers to dangerous areas are urged to prevent mosquito bites by using mosquito repellant and covering exposed skin.

The aedis aegyptia mosquito, which transmits the disease, bites all day long, so individuals need to reapply repellant and not let their guard down, officials warn.

Aedes albopictus mosquitoes, which are found throughout the U.S. and are known for transmitting dengue fever and chikungunya, may also transmit the virus, the CDC said.

Symptoms of the virus include fever, rash, joint pain and red eyes and can last from a few days to about a week. But 80% of individuals infected have no symptoms.

Reported cases spread

Three British travelers have been infected with the virus, health officials said over the weekend.

“As of January 2016, three cases associated with travel to Colombia, Suriname and Guyana have been diagnosed in UK travelers,” Public Health England said on its website.

The government agency did not provide further details about the cases but added that the virus “does not occur naturally in the UK.”

Olympics impacted

Brazil’s Olympic and Paralympic venues will be inspected daily during the 2016 Games in Rio Janeiro, games spokesman Phil Wilkinson said Sunday.

The Rio 2016 organizing committee is in regular contact with the Brazilian Ministry of Health and the Municipal Health Department, which issue guidance on health issues in Brazil and Rio, according to Wilkinson.

The organizing committee will follow virus prevention and control measures provided by the authorities and provide relevant guidance to all who attend the games, the spokesman said.

He noted that the Rio Games will take place during the winter months of August and September when the dryer, cooler climate significantly reduces the presence of mosquitoes.

Zika fever was first discovered in Uganda in the 1940s and has since become endemic in parts of Africa.