The mosquito-borne Zika virus is likely to spread to all countries in North and South America where a specific mosquito species — known as the Aedes mosquito — is present, according to a new statement from the Pan American Health Organization (PAHO).

The PAHO, which is part of the World Health Organization (WHO), said the illness, which has been linked to a fatal birth defect, has now spread to 21 countries and territories throughout the Americas as of Jan. 23.

Aedes mosquitoes, main spreader of the illness, are present in all countries in North and South America except Canada and Chile, the PAHO said in a statement on Jan. 24.

The rapid spread of the virus, which generally only causes mild symptoms, can be attributed to the fact that people in countries such as Brazil had not previously been exposed to the virus.

Zika is a type of arbovirus and is spread through mosquito bites. Its symptoms tend to be mild, lasting from several days to a week, according to the CDC. The most common symptoms are fever, rash, joint pain and conjunctivitis (red eyes).

The disease was first detected in humans in Uganda decades ago, but there had never been reports of links between the virus and brain malformations until recent months, which may be because past outbreaks were small and isolated.

The Aedes species of mosquito spreads the virus, which has turned up in countries across Africa, Southeast Asia and the Americas. This mosquito species also spreads the virus chikungunya, which has become a growing public health concern in the Americas in recent years, and West Nile Virus, which has expanded across the U.S.

The lack of immunity, therefore, is aiding in the spread of the virus. This contrasts with the populations of many African nations, which have had more exposure to Zika and developed immunity over the years.

While only one in five people infected by Zika virus show symptoms, there is increasing concern that the disease can cause microcephaly, a condition that causes smaller than average heads and underdeveloped brains in newborns. For that reason, the CDC as well as several foreign authorities have warned of the risks to pregnant women.

“PAHO anticipates that Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found,” the PAHO said in a statement.

While cases have been reported in the U.S., they have all involved people who had recently traveled to places where active virus transmission was occurring.

The CDC has issued a travel advisory warning pregnant women not to travel to countries and territories where active Zika virus transmission is taking place, including Brazil, where a dramatic escalation in the number of babies born with microcephaly has doctors especially concerned.

Zika is also suspected to be associated with other complications, including autoimmune and neurological disorders such as Guillain-Barre syndrome.

According to Brazilian health authorities, more than 3,500 microcephaly cases were reported in Brazil between October 2015 and January 2016, the CDC says. That’s up from only about 200 such cases in a typical year.

The best defenses against the disease focus on mosquito-control, as well as taking action to prevent being bitten by mosquitos.

The PAHO recommended that people living in or visiting areas with Aedes mosquitoes should use insect repellent; wear light-colored clothes that cover as much of the body as possible; use physical barriers such as screens, closed doors and windows; and sleep under mosquito nets, especially during the day when the mosquitoes are most active.

The CDC’s travel advisories currently apply to: Barbados, Bolivia, Ecuador, Guadeloupe, Saint Martin, Guyana, Cape Verde, Samoa, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela and the Commonwealth of Puerto Rico.

The CDC’s recommendations for women who are pregnant or trying to become pregnant are:

•Pregnant women in any trimester should consider postponing travel to the areas where Zika virus transmission is ongoing. Pregnant women who must travel to one of these areas should talk to their doctor or other healthcare professional first and strictly follow steps to avoid mosquito bites during the trip.

•Women trying to become pregnant should consult with their healthcare professional before traveling to these areas and strictly follow steps to prevent mosquito bites during the trip.

•While the likelihood that active virus transmission will eventually occur in the U.S., that does not mean that there will be a large outbreak in America. For example, outbreaks of dengue and chikungunya have occurred in several states, but have been relatively small.

The travel advisories could spell terrible news for the tourism economies of these countries, in particular Brazil, which is hoping for a surge in tourism for the 2016 Olympics to help with a deep recession. Brazil has deployed the military to assist with mosquito eradication programs in order to try to minimize the health threat from the virus during the Olympic games in August.