Updated July 29, 2:57 pm ET: Florida Gov. Rick Scott announced Friday morning that health officials have ruled out travel and sexual transmission as possible routes of transmission for four Florida people infected with Zika virus.
"This morning we learned that four people in our state likely have the Zika virus as a result of a mosquito bite. This means Florida has become the first state in our nation to have local transmission of the Zika virus," he said at a news conference.
Scott said active transmission is likely occurring in only one small area — about 1 square mile — north of downtown Miami. The four cases appear to have been infected at workplaces in the area.
The state hasn't yet found Aedes species mosquitoes carrying the virus, but is actively testing in the area, Scott said. And the state is contracting with pest control companies to ramp up pesticide spraying.
In order to avoid a contaminated blood supply, the Food and Drug Administration asked Thursday that blood centers in two South Florida counties, Miami-Dade and Broward, cease collecting blood donations until they have the capacity to test each unit for the virus.
People in the impacted area have been advised to contact their county health department if they want to be tested. There are no plans to limit travel to the area, a statement from the federal Centers for Disease Control and Prevention said Friday.
"Florida currently has the capacity to test 6,609 people for active Zika virus and 2,059 people for Zika antibodies," said Scott. "If we need more test kits, we will immediately request them from the CDC."
The state's first travel-related case of Zika virus was identified in February.
State officials said that, based on experience containing dengue and chikungunya outbreaks, they're confident they'll be able to contain the virus.
"We have a very sophisticated, mature control, surveillance and response program throughout the local governments," said Adam Putnam, the state's commissioner of agriculture, adding that the presence of screens, air conditioning and higher standards of living in the U.S. make it unlikely that the virus will spread as it has in other countries.
The reason behind the intense focus on such a small area has to do with the insect that spreads Zika.
"There is a huge difference between mosquito control for West Nile and mosquito control for Zika. With Zika, it's a very focal disease," CDC director Tom Frieden told reporters Friday afternoon. "The Aedes aegypti mosquito does not travel more than about 150 meters in its lifetime, and often quite a bit less than that. In contrast, with West Nile, you have virus circulation within a bird and mosquito population that can cover a large area. That's a totally different situation," he said.
Scott has allocated $26.2 million in emergency funding toward Zika virus prevention and control. He said Friday it's "disappointing" that Congress has so far failed to decide on federal emergency funding to control the virus, and said that the White House promised to send $5.6 million to support state efforts.
"This is not just a Florida issue. It's a national issue. We just happen to be at the forefront," said Scott. He has called on Floridians to wear bug repellent and dump out standing water around their homes in an effort to reduce the risk of infection.
As of July 27, 1,658 cases of Zika have been reported in the continental United States and Hawaii, according to the CDC. None of those were the result of local spread by mosquitoes. Fifteen were believed to be caused by sexual transmission and one was the result of laboratory exposure.
Frieden said he would not be surprised if more cases of Zika virus in the area surface during community surveys, due to the delay between infection from a mosquito bite and when symptoms set in. The CDC and Florida health department expect that mosquito control efforts in the last week would have limited chances for more people to become infected.
But, he said, "If we were to see cases in this area in people infected after the mosquito control efforts were undertaken, this would be a concern and warrant further advice and action."
For continuing coverage of Zika virus, check out our Zika Virus: What Happened When timeline.