A person in Texas has become infected with the Zika virus after having sexual contact with an infected person returning from a country where the virus is present, the health department of Dallas County announced today.

The Centers for Disease Control and Prevention (CDC) has confirmed the case to Medscape Medical News, saying "it is the first US case of Zika virus infection in a non-traveler in the continental US."

The CDC told Medscape Medical News that "there was no risk to a developing fetus" in the Texas case.

"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 said. "We do not have definitive information on the infectious time period, and will provide more guidance for individuals and clinicians as we learn more.

"Sexual partners can protect themselves by using condoms to prevent spreading sexually transmitted infections. People who have Zika virus infection can protect others by preventing additional mosquito bites."

The Dallas County Health and Human Services said in a news release that it has not received any reports of mosquitos transmitting the virus in Dallas County. "However, imported cases make local spread by mosquitos possible," according to the agency. In other words, a mosquito could bite an infected person, acquire the virus, and then bite someone else.

Until now, the only Zika cases reported in the United States recently involved people who had visited countries and territories in Latin America and the Caribbean, where mosquitos are spreading the virus. The CDC reported a possible case of a man spreading the virus to his wife through sexual contact in Colorado in 2008 after becoming infected in Senegal. Although the wife tested positive, no one checked to see if the virus was in the man's semen, which would be evidence of sexual transmission. The Texas case appears to be the first endemic case of Zika infection on US soil during this recent outbreak.

The news from Texas comes just 1 day after the World Health Organization declared that a possible link between the Zika virus and a surge of babies in Brazil born with microcephaly is a global public health emergency. The Centers for Disease Control and Prevention advises pregnant women to postpone visiting countries and territories in the Zika zone.