Costa Rica reported its first case of Zika virus infection Tuesday as the mysterious virus spreads across the Americas, and U.S. officials added the U.S. Virgin Islands and the Dominican Republic to a growing list of countries that pregnant women might want to avoid.
That makes 24 countries on the list, and the World Health Organization predicts the virus will eventually end up in virtually every Western Hemisphere country.
U.S. health officials briefed President Barack Obama on what they do and don't know about the virus and its spread. "The President was briefed on the potential economic and developmental impacts of the Zika virus spreading in the Western Hemisphere," the White House said in a statement.
There was little worry about Zika's spread when it first arrived in Brazil last year. The virus doesn't cause any symptoms in most people and when it does, they are mild - a rash, fever and runny eyes.
But now Brazil has reported a big increase in cases of a birth defect called microcephaly and evidence is adding up that links the virus to the condition. "For the women involved that is very, very serious," said Dr. Tony Fauci, head of the U.S. National Institute of Allergy and Infectious Diseases.
Microcephaly is marked by a smaller-than-average brain and head. Depending on how bad it is, it can cause a miscarriage or stillbirth. Babies who survive are disabled - often very seriously.
Travelers are bringing the virus to the U.S., although it's not expected to spread widely in the U.S. anytime soon.
The Arkansas Department of Health is the latest U.S. state to report a case. New York, Illinois, Florida and Hawaii have also reported cases. The virus is carried by Aedes aegypti mosquitoes - a species more common in the tropics although it's found in warmer U.S. states.
"With current outbreaks in the Americas, cases among U.S. travelers will most likely increase," Ingrid Rabe, a medical epidemiologist, told doctors in a telephone briefing Tuesday.
"With no vaccine or treatment currently available to prevent or treat Zika infection, the best way for individuals—and pregnant women in particular—to protect themselves is to avoid traveling to places where Zika is known to be spreading," Dr. Francis Collins, director of the National Institutes of Health, said in a blog post Tuesday.
United Airlines said it would refund or waive change fees for people who cancel trips to affected areas.
American Airlines had a more limited policy. "We will allow a customer to receive a refund if they provide a doctor's note, stating that they are unable to travel to one of the following cities due to their pregnancy: San Salvador; San Pedro Sula; Tegucigalpa; Panama City; Guatemala City," the airline told NBC News.
And Grupo LATAM, Latin America's largest airline, said it would not charge cancellation or flight-change fees for pregnant women who want to cancel flights to affected countries.
For the virus to move to a new place, a person with active infection must be bitten locally by an Aedes aegypti mosquito and then that mosquito has to bite someone else. The more people who are infected, the more likely the virus is to spread. But in cold weather such spread is unlikely.
So while the CDC says doctors should be on the lookout for the virus, the emphasis for U.S. residents is on travel.
Brazil said it would roll out more than 200,000 servicemembers to help clean up mosquitoes in the worst-affected areas. With Carnival coming up and the Olympics next summer, Brazilian officials are worried that visitors will be scared away from coming.
This story originally appeared on NBCNews.com