A single incident may seem trivial. But until early this year, there was only one known instance of sexual transmission of the Zika virus — a 2008 case in which a mosquito researcher just back from Africa infected his wife in Colorado.

Now scientists believe that sexual transmission is an important driver of the Zika epidemic in the Americas. Cases have been reported in 10 countries where no mosquitoes carry the virus, including France, Germany, Italy, Portugual and New Zealand.

In the French case, a 46-year-old man returned to Paris from Rio de Janeiro on Feb. 10, shortly after experiencing Zika symptoms in Brazil — fever, headache and a rash — that had just ended when he reached France.

He and a 24-year-old partner had sex seven times between Feb. 11 and Feb. 20, each involving vaginal sex without ejaculation and oral sex with ejaculation.