The US Centres for Disease Control and Prevention (CDC) has announced that it's investigating 14 new cases where it appears the Zika virus has been sexually transmitted - worryingly, "several" of those cases involve pregnant women, according to the reports.

While testing is ongoing in these cases, the CDC has admitted that the virus might be transmitted sexually more often than originally thought, and has advised women to either avoid sex or use condoms diligently with men who have recently returned from Zika-affected areas until more information comes to light.

To be clear, the primary method of Zika virus transmission is still overwhelmingly through the bite of Aedes aegypti mosquitoes in the 34 outbreak-affected countries and states, most of which are in the Americas and the Caribbean. 

But although scientists were aware that Zika could be transmitted sexually, it was thought to be an incredibly rare occurrence - until recently, the only confirmed case was in 2008, when a man became infected in Senegal and then passed the virus to his partner after he returned home to Colorado. Another study in Tahiti confirmed the presence of Zika in a man's sperm. 

It wasn't until a few weeks ago, when the first case of sexually transmitted Zika in the US was reported in Texas, that authorities began to question whether sexual transmission was more than a one-off.

The latest 14 cases all involve men who had travelled to Zika-affected areas, and whose female sexual partners later fell ill with Zika-like symptoms - even though they hadn't travelled to outbreak areas themselves. It's thought for now that the virus can only be passed from males to females, and not vice versa.

"In two of the new suspected sexual transmission events, Zika virus infection has been confirmed in women whose only known risk factor was sexual contact with an ill male partner who had recently traveled to an area with local Zika virus transmission," the CDC explained. "Testing for the male partners is still pending."

There's still a whole lot of uncertainty over how sexual transmission would work, and the CDC is now researching important questions, such as how long Zika can stay present in semen, when men are the most infectious, and which sex acts can pass the virus on.

But according to the CDC, this is what we know so far (and this information might change as we find out more):

  • Zika virus can be spread by a man to his sex partners.
  • In known cases of likely sexual transmission, the men had Zika symptoms.
  • In one case, the virus was spread a few days before symptoms developed.
  • The virus is present in semen longer than in blood (a pre-print article released this week suggests that Zika was found in the semen of a patient 62 days after he became sick).

Despite not having all the facts just yet, the CDC has put in place interim guidelines that recommend men who've recently travelled to outbreak areas - particularly those who have pregnant partners - to either abstain from sex, or ensure condoms are used during vaginal, anal, and oral sex.

"We are not changing the guidance today but we are really reinforcing it," said Anne Schuchat from the CDC. "For the time being we are telling women to avoid sex or to be careful during sex with a partner who is coming back from an area where Zika is."

"The science is not clear on how long the risk should be avoided. Research is now underway to answer this question as soon as possible," the CDC added.

The virus is already strongly suspected to be linked to microcephaly - a birth defect that causes babies to be born with abnormally small brains and skulls. But these increased guidelines come just weeks after suggestions that the virus might also be linked to a condition called Guillain-Barre syndrome, which can cause muscle weakness and paralysis.

There's also evidence that Zika could eventually be spread by the Asian tiger mosquito (Aedes albopictus), which is more prevalent in the US.

Neither of those hypotheses have been confirmed just yet, but health officials are understandably worried - there's currently no vaccine against Zika, and despite the extent of the outbreak, it's becoming apparent that we're still in the dark about exactly how it spreads.

For now, it pays to be cautious. But rest assured that the real threat here is still mosquitoes. "It's not likely that sexual transmission is anywhere close to the frequency of mosquito-borne transmission," infectious disease expert William Schaffner from Vanderbilt University told NBC. "The mosquito is the most dangerous animal on the planet."