ARI SHAPIRO, HOST:
The Centers for Disease Control and Prevention has expanded the list of countries that pregnant women should avoid because of the Zika virus. Health officials say the mosquito-borne illness may cause severe birth defects. In Brazil, where there's been an outbreak, thousands of babies have been born with brain damage. As NPR's Jason Beaubien reports, researchers say those cases may be just the beginning.
JASON BEAUBIEN, BYLINE: Something new and quite frankly frightening appears to be happening with the Zika virus right now. For decades, Zika was a virus that turned up in monkeys and occasionally in humans in Africa and Southeast Asia, but its symptoms were mild and the number of confirmed human cases was extremely low. But in 2007, there was a big Zika outbreak in the island of Yap in Micronesia. Seventy-five percent of the island's population got infected, or about 5,000 people. Still, few people reported any symptoms at all. Then in October of last year, babies started turning up with smaller than normal heads in Brazil.
ALBERT KO: We do still know so little about this virus and the harm that it can cause. This is really a relatively new pathogen.
BEAUBIEN: Albert Ko, an epidemiologist from Yale, has been working with the Brazilian Ministry of Health to investigate the Zika outbreak. The big question now is, why all the sudden is this virus spreading like wildfire? Ko says it may be that the Americas are fertile ground for the virus. This hemisphere has the right type of mosquitoes to transmit Zika and people have no immunity to it. Or, it may be that the virus has mutated.
KO: This may be a new strain that's traveling very quickly that's been able to adapt itself to the mosquito. But we really don't know. We have to do more - you know, there's more work that needs to be done.
BEAUBIEN: The CDC has issued a travel alert for pregnant women encouraging them to avoid 22 tropical countries where Zika transmission has been reported. Officials in Brazil and Colombia have taken the extraordinary step of asking women to not get pregnant. The link between Zika and microcephaly, a condition where a baby's brain and head don't fully develop, still hasn't been definitively proven but health officials are operating under the assumption that there is one. Brazil reported only 150 cases of microcephaly in 2014 before the virus arrived yet has recorded nearly 4,000 cases of the birth defect since October.
Marcie Treadwell is a fetal medicine physician at the University of Michigan in Ann Arbor. She says it's possible that in many more babies the virus is causing less severe forms of brain damage that will only become apparent later.
MARCIE TREADWELL: We don't know the full range of the impact of the virus.
BEAUBIEN: Given that many people who get infected with Zika don't get sick at all, she says...
TREADWELL: There may be a whole host of women who have had the virus while pregnant who have kids who are completely normal or that maybe appear completely normal that develop mental issues as these kids get older and we start to see some of the impact. We just don't know the range.
BEAUBIEN: Currently there's no treatment for Zika nor is there a vaccine. A half dozen cases of Zika have already turned up in the U.S. over the last two weeks, all of them in travelers returning from parts of Latin America and the Caribbean, where the outbreak is raging. CDC officials, however, predict any outbreaks in the U.S. mainland will be localized and relatively small because Americans have better access to screens and air conditioning to protect themselves from mosquitoes. Jason Beaubien, NPR News.