The first case of female-to-male sexual transmission of the Zika virus has been found, the Centers for Disease Control and Prevention reported Friday.

The finding has led the CDC to recommend that pregnant women who have sex with women who might be infected with Zika use protection or do not have sex during the duration of the pregnancy. Being infected while pregnant can cause the fetus to develop microcephaly, a birth defect resulting in abnormally small heads and likely brain damage.

No female-to-female sexual transmission of Zika has been reported.

The agency did not identify the woman other than to say she was in her 20s and lived in New York City. After returning from an area of the world where Zika is spreading, she had vaginal sex with an uncircumcised man in his 20s who did not use a condom.

As she had been returning to New York, the women had pain in her abdomen and had a headache. The day after she had sex, she had a fever, fatigue, a rash and body pain – all typical signs of adults who are infected with Zika. She also reported that she had a heavier period than usual.

She went to see a doctor the following day, and the virus was found in her blood and urine.

Seven days after the couple had sex, the man developed a fever, joint pain, a rash and conjunctivitis. Three days after his symptoms stated, he sought care from the same doctor his female partner went to see. The doctor, suspecting Zika, alerted the city's public health department and tested his urine and blood. The test showed Zika in his urine.

CDC officials say that the woman was likely already infected with Zika when she had sex, and that the virus was probably present in her vaginal fluids or menstrual blood. They believe it may have spread when she was exposed to her partner's fluids or undetected abrasions on his penis.

The couple reported they didn't bleed during or following intercourse, and did not have oral or anal sex.

Recent studies have already shown that Zika is present in the female genital tract, including in vaginal fluid, but this is the first recorded case of transmission.

Scientists are still learning about the Zika virus, which is mostly spread through mosquitoes but can also be transmitted sexually. The CDC had previously reported that men who have sex with men can spread Zika to their partners through anal sex and to their female partners through vaginal or anal sex. Previous public health guidance had advised men who travel to Zika-infected areas wear condoms when they have sex with their partners.

CDC officials on Thursday requested that men who have had Zika consider donating their semen so scientists could learn more about how the virus stays in the body.