A family member holds twins Eloisa (left) and Eloa, both 8 months old and born with microcephaly, during a Christmas gathering. The mother of the twins, Raquel, who lives in Brazil, said she contracted Zika during her pregnancy. Mario Tama/Getty Images hide caption
Yalieth Gonzalez and her baby Micaela Delgado at the Maternal Infant Studies Center at the University of Puerto Rico. Marisa Penaloza/NPR hide caption
The National Institutes of Health is based in Bethesda, Md., but the vast majority of its research funding flows to universities around the country. NIH/Flickr hide caption
Andie Vaught grasps a stress toy in the shape of a truck as she prepares to have blood drawn as part of a clinical trial for a Zika vaccine at the National Institutes of Health in Bethesda, Md., in November 2016. Allison Shelley/The Washington Post/Getty Images hide caption
From left: A scene from the video of a car crash test; illustration of a mosquito transmitting the Zika virus; a menstrual shed in Nepal. Insurance Institute for Highway Safety/YouTube; BSIP/UIG via Getty Images; Poulomi Basu/Magnum Emergency Fund hide caption
No one is sure why the northeastern part of Brazil has been hit so hard with microcephaly cases. Felipe Dana/AP hide caption
Florida Department of Health workers package a urine test, part of the state's effort to provide free Zika tests to pregnant women. Joe Raedle/Getty Images hide caption
A mother holds her baby, who has microcephaly, in Recife, Brazil. Mario Tama/Getty Images hide caption
Sudanese dockers unload a U.S. aid shipment at Port Sudan on the Red Sea coast on May 5. Ashraf Shazly/AFP/Getty Images hide caption
The Zika virus has been largely spread by mosquitoes, but it can also be spread by sexual intercourse. NIAD/Flickr hide caption
A strain of the Zika virus is stained green in this image from a transmission electron microscope. The virus sample came from a microcephaly case in Brazil. Science Source hide caption
A government worker sprays mosquito insecticide fog in Kuala Lumpur, Malaysia, earlier this month to block the spread of Zika. The U.S. CDC advises pregnant women to reconsider plans to travel to Malaysia and 10 other countries because of the virus. Joshua Paul/AP hide caption
A health department microbiologist looks for mosquitoes carrying Zika virus in Hutchins, Texas. LM Otero/AP hide caption
Infants born with microcephaly are held by mothers at a meeting for mothers of children with special needs in Recife, Brazil. Mario Tama/Getty Images hide caption