Thirty-three members of the U.S. military have contracted the Zika virus while serving overseas, Pentagon officials said.

One of the service members is a pregnant woman, CNN reported. Health officials have linked Zika to severe birth defects in infants, leading to greater concern for pregnant women with the virus.

Maj. Ben Sakrisson, a Pentagon spokesman, said an additional six people who are family members of military personnel have also contracted the virus, according to CNN. Officials said the service members are in countries where the virus has been identified, though they declined to specify where or how many people are still ill.

The Army is working with partners to develop a Zika virus vaccine, CNN reported. The National Institutes of Health announced Wednesday that 80 volunteers will test a new Zika vaccine in a human trial. The NIH reports that there are more than 6,000 Zika cases in the continental U.S. and U.S. territories.

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Write to Katie Reilly at Katie.Reilly@time.com.