Public-health officials in Latin American and Caribbean countries are warning their citizens against pregnancy amid rising fears that the Zika virus, which may be linked to a rise in birth defects, is spreading through the region.

The worry about Zika comes from a suggested link between the virus and microcephaly, a birth defect where babies are born with smaller-than-normal heads and incomplete brain development. While it has not been proven that Zika causes microcephaly, the CDC found the virus in the tissue of four Brazilian infants with the birth defect.

Zika is mosquito-borne and historically found in Southeast Asia and Africa. The first case in Brazil was reported in May 2015, and the virus has since spread around the region. Health officials in Brazil saw 20 times more cases of microcephaly in 2015 compared to 2014.

Countries across the affected region are coming up with various ways to protect against the virus. In El Salvador, which has seen around 5,000 cases of Zika, officials are urging women not to get pregnant until 2018. Critics of the strategy worry about pregnancies that are unplanned.

Jamaica, Colombia, and Ecuador have also recommended that their citizens delay pregnancy. In Colombia, the suggested delay is currently six to eight months. Colombian officials raised their estimate for suspected cases on Tuesday, saying more than 16,000 people in the country have the virus. More than 1,000 of them are pregnant women, according to the AP.

Brazil, the country worst hit so far, has reported more than 1 million cases. The Brazilian government has sent soldiers “door to door” to destroy places where mosquitoes could breed. The New York Times reported Wednesday that Brazil plans to deploy 220,000 troops for a single day next month to raise awareness about the virus.

In the U.S., several cases of Zika have been reported in Florida, Illinois, Arkansas, Texas, Hawaii, California, and Virginia. In Hawaii, a woman who spent part of her pregnancy in Brazil recently gave birth to a baby with microcephaly. While all cases in the U.S. so far are among people who were traveling and thus likely infected while abroad, the WHO has warned the virus could spread north.

The CDC issued travel alerts for Mexico, Puerto Rico, and countries in the Caribbean and Central and South America, urging travelers to take extra precaution against mosquitos while in the area. The most recent countries to join that list are the U.S. Virgin Islands and the Dominican Republic, the AP reported Tuesday. The agency also recommended that pregnant women avoid all travel to a current total of 22 countries in the region.

The virus, which is transmitted through the bite of infected mosquitos of the Aedes genus, was first identified in humans in 1952. Until recently, there was little cause for concern, because the symptoms of the virus are mild, according to the World Health Organization. The WHO advises that Zika “requires no specific treatment,” and says there is also no vaccine for the virus.