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By Maggie Fox

The Zika virus is hitting in women of childbearing age hardest in Puerto Rico — bad news for a virus most dangerous to developing babies, researchers reported Wednesday.

The findings raise questions about the role sexual transmission plays in spreading the virus, which is mostly carried by mosquitoes, the researchers said.

A flyer that reads, "Questions and answers: Zica virus infection during pregnancy" is seen posted outside a doctor's office, at a public hospital in San Juan, Puerto Rico, February 3, 2016.?(C) Alvin Baez / Reuters / Reuters file

Zika virus is spreading at epidemic rates in Puerto Rico, where more than 30,000 cases have been reported, the Centers for Disease Control and Prevention says. Many more cases go unreported.

The CDC's Matthew Lozier and colleagues dissected confirmed and reported cases of Zika across the U.S. island territory. "The highest incidence among confirmed or presumptive cases occurred among per­sons aged 20–29 years," they wrote.

"It is possible that male-to-female sexual transmission is a contributing factor to this skewing of the burden of disease toward women."

More than 60 percent of cases were in women, they found. It's possible that pregnant women are more likely than men to seek treatment for Zika because of the potential risk of birth defects — but even when the team excluded pregnant women, more than 50 percent of cases were in females.

"It is possible that male-to-female sexual transmission is a contributing factor to this skewing of the burden of disease toward women," the CDC said in a statement.

Related: This Study Removes Any Doubt Zika Causes Birth Defects

"Similar observations have been made in Bahia state (Brazil) and El Salvador, where, overall, the reported incidence of clinically suspected Zika virus disease cases was 75 percent higher in females than in males," the researchers added.

"Although male-to-female and female-to-male sexual transmission has been documented, data from Rio de Janeiro suggest that differences in infection rates between men and women might be explained by male-to-female sexual transmission. The same explanations might be responsible for the observed trends in Puerto Rico."

Another recent study looked at the Aedes mosquitoes that spread Zika. Researchers know they like to lurk in and around homes, and researchers in Bangladesh found women who spent more time in homes there were more at risk from chikungunya, a virus spread by the same mosquitoes.

Related: Zika Killed This Woman's Baby

Zika virus causes a range of birth defects, including microcephaly, marked by a very small head, as well as other types of brain damage and defects in the eyes and limbs. There is no cure.

Zika virus can also cause Guillain-Barré syndrome, a rare paralyzing disorder, and other central nervous system inflammation, and it has also caused a blood disorder called thrombocytopenia.