This case in Dallas would be the first known infection to take place in the mainland US, though Texas has seen seven other Zika cases all related to foreign travel.
Anne Schuchat, principal deputy director for CDC, said this was the first case it had dealt with involving a "non-traveller".
"We don't believe this was spread through mosquito bites, but we do believe it was spread through a sexual contact."
A statement issued by the CDC said the best way to avoid Zika virus infection was "to prevent mosquito bites AND to avoid exposure to semen from someone who has been exposed to Zika".
The case is "significant" if it was definitely transmitted through sexual contact, Alaka Basu, a senior fellow for public health at the UN Foundation, told the BBC.
"This significance is parallel with the HIV/Aids case. It's worse in some ways, because there are two modes of transmission."
It is not the first known case of sexual transmission. There was a case in 2013 in French Polynesia.
Zika virus infection 'through sex' reported in US - BBC News
Anne Schuchat, principal deputy director for CDC, said this was the first case it had dealt with involving a "non-traveller".
"We don't believe this was spread through mosquito bites, but we do believe it was spread through a sexual contact."
A statement issued by the CDC said the best way to avoid Zika virus infection was "to prevent mosquito bites AND to avoid exposure to semen from someone who has been exposed to Zika".
The case is "significant" if it was definitely transmitted through sexual contact, Alaka Basu, a senior fellow for public health at the UN Foundation, told the BBC.
"This significance is parallel with the HIV/Aids case. It's worse in some ways, because there are two modes of transmission."
It is not the first known case of sexual transmission. There was a case in 2013 in French Polynesia.
Zika virus infection 'through sex' reported in US - BBC News