Dallas County health officials say the case of two people infected with the Zika virus there almost certainly constitutes sexual transmission — something health experts knew was possible but which has only been seen to happen once.
But the incident raises even more questions about the already mystifying virus, which for decades seemed to be about as harmless and inconsequential as a virus can be until it started being blamed for a surge in severe birth defects in Brazil. Here are five that even experts are asking:
Is sexual transmission common?
Until Tuesday's case in Dallas, only one case of certain sexual transmission of Zika had ever been documented: a researcher infected in Senegal who came home to Colorado and infected his wife in 2008. But scientists knew it was theoretically possible, if rare.
"Everything we know about Zika suggests that the overwhelming majority of cases are spread by mosquitoes," Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention, told NBC News.
But of course, most people who have sex live in the same place, so it would be almost impossible to tell whether someone was infected by a sex partner or by the same mosquitoes or mosquito. The only way to really tell is in cases where an infected person travels to an area where no one else has the virus and gives it to someone else.
Tests on two men in the past show the virus can be found in semen. It's not certain that is how it would spread sexually, but likely. There are viruses that spread in saliva and even in the air, but Zika isn't one of them.
Does that mean it's more likely to spread even more?
Experts say probably not but again, it is almost impossible to say for sure.
Zika infections usually clear up after about a week, and the odds of many travelers coming home while infected and having sex are fairly low. Most experts believe Zika could only be sexually transmitted while a person is actively infected.
Frieden says it's not like the AIDS virus HIV, or herpes, which stay in the body for life.
There may be a few cases of travel-related sexual transmission but for any virus to spread widely and quickly, there must be many vectors spreading it. In the Zika zones that's the Aedes aegypti mosquito, not other people.
Could this mean Zika's more dangerous than we thought?
The answer has to be that experts don't know.
Probably not, but as National Institute of Allergy and Infectious Diseases director Dr. Tony Fauci says: "Never say never and never say always." Viruses can and do change in surprising ways and when a virus starts to infect millions of people instead of just a few thousands, events that previously were too rare to even notice will start to become more common and visible.
The severe acute respiratory syndrome (SARS) virus that caused a brief epidemic and killed nearly 800 people in 2003, and the Middle East Respiratory Syndrome (MERS) virus that is still spreading in the Arabian peninsula area both are coronaviruses, a family of viruses that had also been believed to be relatively harmless to people.
That said, scientists do know that viruses don't mutate wildly. A virus like Zika that is transmitted by mosquitoes will not mutate into a form that could be airborne, like measles virus.
Did something happen to make the virus different?
This is also not clear.
There is a little bit of evidence that one gene on the virus has mutated, or changed slightly. Viruses only have a few genes to start with, so a change in one can greatly affect how it behaves. Because Zika is very difficult to test for, it will take some time to get answers to those questions.
Zika's also circulating in a region where two other new viruses are also circulating: dengue and chikungunya. They are all somewhat related and they are all spread by the same mosquito. Scientists are trying to figure out if the viruses may be interacting in some way to produce an unusual pattern of disease or spread.
Human genetic factors may be important, also. There's evidence that people from the same ethnic groups, because they happen to share certain genes, may have the same susceptibility to certain infections.
What else is it doing that we didn't know about?
That's a big question.
There are no on-the-spot tests for Zika so researchers are fanning out across Brazil and other affected areas to try to catch people who are actively infected, get blood samples, test them and then save them to see what happens later. Zika usually clears from the body days or weeks after infection so it's very hard to test someone now to see if they were infected years or even months ago.
And because Zika doesn't cause symptoms in 80 percent of people who have it, it flies very much under the radar.