Florida health officials said Thursday they were investigating a second possible case of Zika spread locally, and Brazilian scientists said they feared they may have found a second species of mosquito can transmit the virus.
The two Florida cases — one in Miami-Dade county and another in Broward county — both appear to have no connections to travel to Zika-affected areas, and neither appears to have had sexual contact with a Zika-infected patient, but Florida officials are still checking both possibilities.
They're also looking for mosquitoes infected with Zika near both homes, and testing people in both areas to see if anyone else may have been infected with Zika and not known it.
"Residents and visitors are urged to participate in requests for blood and urine samples by the department in the areas of investigation. These results will help the department determine the number of people affected," the Florida Department of Health said in a statement.
The Centers for Disease Control and Prevention says it has official reports of 1,404 cases of Zika in the continental United States — all travel related.
But the CDC says it's very likely that some travelers will be bitten by mosquitoes while still actively infected and that this could cause local Zika outbreaks.
That's what Florida officials are checking for now.
But it takes two factors for a local outbreak: An infected patient, and an Aedes mosquito that bites the person and then lives long enough for the virus to build up in its body before it bites someone else.
Finding infected mosquitoes isn't necessarily easy, said Dr. Peter Hotez, dean of tropical medicine at the Baylor College of Medicine in Houston.
"It's a needle in a haystack," Hotez said. Mosquitoes don't transmit the virus to one another, so to find Zika-affected mosquitoes, workers must catch a mosquito that actually bit someone who was infected.
"Just because they don't find Zike in an Aedes mosquito doesn't mean there is no transmission," he said.
Florida is one of 26 U.S. states where Aedes aegypti mosquitoes have been found.
"Now if they get lucky and find it, that's entirely confirmatory," Hotez added.
At the same time, officials are making sure that there's no other possible way the two Florida patients could have been infected.
"We continue to investigate and have not ruled out travel or sexual transmission at this time," a department spokesperson told NBC News.
Separately, Brazilian researchers said they'd found that a much more common mosquito, a species called Culex quinquefasciatus, has been infected with Zika. The same team of researchers, at Brazil's Oswaldo Cruz Foundation, reported in March that they had infected Culex mosquitoes with Zika in the lab.
Related: Here's How You Find Zika
Now they say they found the virus in live mosquitoes.
If Culex can spread Zika, that would be more troubling. Culex mosquitoes are far more common in temperate zones, such as in the United States.
But just this week another team, led by Scott Weaver at the University of Texas Medical Branch, said they had tried and been unable to infect Culex mosquitoes with Zika.
"Without access to the methods and data (it is not published in a peer-reviewed journal yet) there is not much that I can say about this Brazilian study," Weaver told NBC News by email.
"An effective vector requires not only susceptibility to infection and ability to transmit (virus replication in salivary glands) but a high rate of biting humans in the case of Zika virus."
Hotez also says he doubts Culex mosquitoes play a big role in spreading Zika.
"If they found Zika in the Culex mosquitoes, it could be they had taken a blood meal from someone with Zika and then you would find the virus. It doesn't mean they transmit the virus," Hotez said.
And everywhere so far that Zika has spread have been areas where Aedes aegypti mosquitoes abound.
Different mosquito species spread different diseases. Culex mosquitoes can spread West Nile virus, but so far are not known to spread Zika. Aedes aegypti mosquitoes spread yellow fever, dengue, chikungunya and Zika viruses, which are all closely related and biologically adapted to these mosquitoes.
And malaria, caused by a parasite called Plasmodium, is spread by entirely different species of mosquito, mostly Anopheles.
The big danger from Zika is to pregnant women. It causes severe birth defects in babies if the mother is infected while pregnant.
The CDC says it knows of 400 pregnancies affected by Zika in the continental U.S. and another 378 in territories such as Puerto Rico. Of these, 12 babies have been born with birth defects caused by Zika and another six have died, miscarried or been aborted because of severe defects.