Health officials in Texas have confirmed the first case of Zika virus transmission in the US, and are pointing to sexual transmission as the culprit. The Dallas County Health and Human Services said in a release that the patient engaged in sexual contact with an infected individual who had recently traveled to a country where the Zika virus is spreading. The CDC tells The Verge that it has confirmed the patient is infected with the Zika virus, but that the agency has not confirmed the method of transmission. County officials say there are no reports of the virus being locally transmitted by mosquitoes.

While the CDC is not yet confirming sexual transmission, it is warning people of the possibility. "Based on what we know now, the best way to avoid Zika virus infection is to prevent mosquito bites AND to avoid exposure to semen from someone who has been exposed to Zika virus or has been ill from Zika virus infection," the CDC says. "We do not have definitive information on the infectious time period, and will provide more guidance for individuals and clinicians as we learn more."

The Zika virus can cause fevers, rashes, joint pain, and conjunctivitis, but it only elicits symptoms in about 20 percent of those infected. It is more commonly transmitted through mosquito bites, but it can also be passed from a mother to her child before the child is born. These transmission methods are at the center of the current outbreak in Latin America, which is spreading so fast that the World Health Organization has declared it a "public health emergency." Health officials in Brazil are worried that the spread is tied to a drastic increase in reported cases of microcephaly, which causes abnormally small head sizes in children and stunts development of the brain.

The WHO estimates that, without any intervention, the Zika virus could infect as many as 4 million people in the Americas this year. Classifying the outbreak as a public health emergency is a major step toward curbing the spread, though, as it should trigger funding and resources for the countries that need help to fight Zika.