The World Health Organization (WHO) is now raising concerns about the Zika virus through sexual transmission in the United States. WHO has called for further investigation into the mosquito-borne virus, which has also been linked to birth defects.

The first known case of Zika virus transmission in the continental United States was reported in Dallas on Tuesday, February 2nd, 2016 by local health officials. Dallas County health officials stated that the patient did not travel but was infected after having sexual contact with an ill person who had returned from Venezuela.

“We certainly understand the concern. This needs to be further investigated to understand the conditions and how often or likely sexual transmission is, and whether or not other body fluids are implicated,” WHO spokesman Gregory Hartl told Reuters on Wednesday.