A patient diagnosed with the Zika virus in Texas became infected after having sexual contact with another sufferer, health experts have said.
It marks the first case of sexual transmission of the virus in the US, the Centres for Disease Control and Prevention confirmed.
Dallas County health officials said the patient's partner fell ill after visiting a country where Zika is rife.
While six people in Harris County, close to Houston, have contracted the disease after traveling to affected regions, this is thought to be the first case of a person becoming infected on US soil, having not traveled.
Zachary Thompson, director of Dallas County Health and Human Services, said: "Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others.
"Next to abstinence, condoms are the best prevention method against any sexually-transmitted infections."
Sexual transmission of the virus is very rare.
It was found in one man's semen in Tahiti, and there was report of a Colorado researcher who caught the virus overseas and apparently spread it to his wife back home in 2008.
More often however, sufferers become infected after being bitten by a mosquito carrying the virus.
In the vast majority of cases the virus triggers mild symptoms, including fever, rash, joint pain and conjunctivitis.
However, the World Health Organisation yesterday declared the current outbreak of Zika virus, predominantly in South America, a global public health emergency.
WHO director-general Dr Margaret Chan said she was moved to do so, after growing evidence the virus can trigger potentially life-threatening birth defects in newborn babies.
In assessing the level of threat, Dr Chan said 18 experts and advisers looked in particular at the strong association, in time and place, between Zika infection and a rise in cases of microcephaly - where a baby is born with a small or incomplete brain.
Dr Chan said: "The experts agreed that a causal relationship between Zika infection during pregnancy and microcephaly is strongly suspected, though not yet scientifically proven."
Dallas County Health Services urged any individuals with symptoms to see a doctor if they have visited an area where Zika virus us present.
Furthermore, a spokesman said it is important anyone who has had sexual contact with a person diagnosed with Zika, should seek medical help.
Health officials note there are no reports of Zika being locally transmitted by mosquitoes in Dallas county.
There is no specific medication to treat Zika, and there is currently no vaccine to prevent infection.
The best way to avoid the virus is to avoid being bitten by a mosquito, and to avoid unprotected sexual contact with an infected individual.
Dr Christopher Perkins, DCHHS medical director, said: "Education and awareness is crucial in preventing Zika virus.
"Patients are highly encouraged to follow prevention recommendations to avoid transmitting and spreading Zika virus."
DCHHS recommends the following 4Ds to avoid Zika virus, reducing the chance of being bitten by a mosquito:
• DEET - whenever you are outside, use insect repellents that contain DEET or other EPA approved repellents and follow instructions
• Dress - wear long, loose and light-colored clothing outside
• Drain - remove all standing water in and around your home
• Dusk and dawn - limit outdoor activities during the hours around dusk and dawn when mosquitoes are most active
Travellers can also protect themselves by choosing a hotel or lodgings with air conditioning and screens on windows or doors, and by sleeping under a mosquito net if outside.
Sexual partners can protect each other by using condoms to prevent spreading sexually-transmitted infections.
There are currently no reports of Zika virus being locally-transmitted by mosquitoes in Dallas County.
However, imported cases make local spread by mosquitoes possible because the mosquitoes that can transmit the virus are found locally.
DCHHS advises recent travelers with Zika virus symptoms as well as individuals diagnosed with Zika virus protect themselves from further mosquito bites.
- Daily Mail