The case, announced by Dallas County health officials, involved a patient who had sex with someone who had recently returned from Venezuela infected with the mosquito-borne virus.
In a statement to CNN, the CDC said it confirmed the test results showing Zika present in the blood of a "nontraveler in the continental United States." They stressed that there was no risk to a developing fetus in this instance.
Based on that, the CDC says it will soon provide guidance on sexual transmission, with a "focus on the male sexual partners of women who are or who may be pregnant."
Earlier Tuesday, CDC Director Tom Frieden told CNN's Chief Medical Correspondent Sanjay Gupta: "There have been isolated cases of spread through blood transfusion or sexual contact and that's not very surprising. The virus is in the blood for about a week. How long it would remain in the semen is something that needs to be studied and we're working on that now."
Frieden added that studies on sexual transmission are not easy studies to do, but the CDC is continuing to explore that avenue of transmission. "What we know is the vast majority of spread is going to be from mosquitoes," Frieden added. "The bottom line is mosquitoes are the real culprit here." Read more on edition.cnn.