Health officials in Dallas this week announced that a local had contracted the Zika virus.

It wasn't the first case in the country. At least 30 people in the United States have tested positive for the virus, which has been found in more than two dozen nations around the globe and linked to shrunken, damaged brains in hundreds, perhaps thousands, of newborns worldwide.

The Dallas infection was unique, however, because the virus was transmitted sexually.

"This is a game changer," Zachary Thompson, director of the Dallas County Health and Human Services, told local television station WFAA.

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Almost unheard of a few months ago, Zika is suddenly scaring scientists and dominating headlines across the planet. On Monday, the World Health Organisation (WHO) designated the virus and its suspected complications in newborns a public health emergency of international concern, a nearly unprecedented designation that officials hope will rally a global response.

The news out of Dallas complicates those efforts, which so far has focused on the mosquitoes mostly responsible for spreading the virus.

But it is also a terrifying twist to the already frightening outbreak.

The revelation that Zika can be sexually transmitted comes on the heels of an October announcement that the deadly Ebola virus could live in a man's sperm for up to nine months, enabling sexual transmission long after the patient's apparent recovery.

The two viruses have injected fear back into sex just as worries over HIV appear to be ebbing slightly.

Four months ago, health officials were urging male Ebola survivors in West Africa to use condoms until the virus vanished from their semen. Now, those same officials are asking people in the Americas and elsewhere to practice safe sex lest they, and their unborn children, contract the disease.

The US Centres for Disease Control says it will release new guidelines on preventing the sexual transmission of Zika in the coming days, focusing on male sexual partners of women who are or who may be pregnant.

"There have been isolated cases of spread through blood transfusion or sexual contact and that's not very surprising," CDC Director Tom Frieden told CNN. "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 stressed, however, that "the vast majority of spread is going to be from mosquitoes," and that "the bottom line is mosquitoes are the real culprit here."

Some, however, cautioned that the Dallas case required more study.

Either way, Zika has quickly joined Ebola, HIV and syphilis as serious STDs shaping popular attitudes towards sex.

When syphilis emerged in Europe in the late 15th century, likely carried by conquistadors returning from the New World, the bacteria became a persistent plague until the discovery of penicillin in 1928. Along the way, the scourge helped mold social mores.

And when Ebola ravaged West Africa in 2014, the virus also affected how people saw sex. Officials initially ordered survivors to abstain for 90 days after recovery, only to later up that to nine months.

"There is no more trust between me and my girlfriend," Sierre Leone resident Augustine Mansaray told Salon. "We always quarrel because she is even afraid to come close to me, not to talk of making love."

Many male Ebola survivors faced lasting stigmatisation.

"We've got people being treated horrendously," Margaret Harris, a WHO spokeswoman on Ebola told Fox News last year. "In Sierra Leone male survivors have been put in a form of concentration camp."

The HIV/Aids epidemic had much broader and longer-lasting effects on societal attitudes towards sex. In the US, the crisis coincided with an increase in disapproval for extramarital sex, researchers say, and spurred a rise in abstinence-only education.

Ironically, the news that Zika can be sexually transmitted comes just as pharmaceutical advancements are reducing sexual anxiety surrounding HIV. A blue pill known as pre-exposure prophylaxis, or PrEP, has proven so effective in preventing the transmission of HIV that it has become popular in the Bay Area's gay community, the Washington Post's Ariana Eunjung Cha reports. Many gay men now openly mention that they take PrEP on social media and dating websites. Sex is beginning to emerge from underneath the shadow of HIV.

The revelation that Ebola and now Zika can also be sexually transmitted, however, clouds that picture somewhat.

Officials have been keen to point out that Zika is no Ebola, let alone another HIV. Unlike the other two viruses, Zika is not deadly. Healthy, non-pregnant adults who contract Zika usually encounter symptoms such as fever, pain and itching, although microcephaly linked to Zika can cause debilitating, lifelong brain damage.