An outbreak of the Zika virus in the continental United States could begin any day now. But while there is plenty of discussion about mosquito bites, some researchers are beginning to worry more about the other known transmission route: sex. Intimate contact may account for more Zika infections than previously suspected, these experts say. The evidence is still emerging, and recent findings are hotly disputed. All experts agree that mosquitoes are the epidemic's main driver. But two reports now suggest that women in Latin America are much more likely to be infected than men, although both are presumed to be equally exposed to mosquitoes. The gender difference appears at the age at which sexual activity begins, and then fades among older adults. Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, called the evidence "striking." Like other scientists, he had doubts about aspects of the data, but thought the results justified a more rigorous