WASHINGTON (AP) — Researchers will study some U.S. athletes for Zika during the summer Olympics and Paralympics in Brazil, to better understand how the virus infects.

The National Institutes of Health announced the study Tuesday, amid concern among athletes over the mosquito-borne virus, which can cause severe birth defects.

International health officials have said pregnant women should skip the games, but the virus also sometimes spreads through sex and may persist in semen longer than blood. Some athletes from several countries have dropped out, citing Zika worries.

NIH-funded researchers aim to enroll at least 1,000 athletes, coaches and staff. Participants will provide samples of bodily fluids for routine testing to determine to help determine risk factors for infection, and where and how long the virus persists in the body.