We already know that sexual intercourse can spread Zika virus, the mosquito-borne illness linked with serious birth defects. Now, French researchers are suggesting that Zika may also be transmitted through oral sex, NBC News reports. The scientists at France’s Inserm medical institute detailed an episode they think suggests oral sex may be one of the culprits spreading the disease so quickly among humans. The case began with a 24-year-old woman who got sick in Paris this February. According to researchers, she came into the hospital displaying common early side effects of the virus, including a rash and muscle aches. She also reported having sexual contact with a man who had recently stayed in Brazil and also had a rash. Zika has been spreading across Latin America, the Caribbean and parts of the South Pacific since late 2015. The virus triggered a startling number of cases of microcephaly—a birth defect that causes infants to be born with smaller than normal heads—leading to developmental delays as well as disease-causing damage to nerve cells. As a safety measure, the World Health Organization (WHO) updated its recommendations on pregnancy, sex and the risk of Zika virus earlier this week. The organization advised people returning from Zika-affected areas to use condoms or abstain from sex for at least six weeks to ensure they wouldn’t transmit the virus to others.