ORLANDO, Fla. » Mosquitoes have apparently begun spreading the Zika virus on the U.S. mainland for the first time, health officials said in a long-feared turn in the epidemic that is sweeping Latin America and the Caribbean.

Four recently infected people in the Miami area — one woman and three men — are believed to have contracted the virus locally through mosquito bites, Gov. Rick Scott said at a news conference.

No mosquitoes in Florida have actually been found to be carrying Zika, despite the testing of 19,000 by the state lab. But other methods of Zika transmission, such as travel to a stricken country or sex with an infected person, have been ruled out.

“Zika is now here,” said Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention.

Still, U.S. health officials said they do not expect widespread outbreaks in this country of the sort seen in Brazil, in part because of better sanitation, better mosquito control and wider use of window screens and air conditioners.

The virus has triggered alarm across the warmer latitudes of the Western Hemisphere. While most people who get Zika don’t even know they are sick, infection during pregnancy can cause babies to born with disastrously small heads and other severe brain-related defects.

More than 1,650 people in the mainland U.S. have been infected with Zika in recent months. The four people in Florida are believed to be first ones to contract the virus within the 50 states from mosquitoes.

“This is not just a Florida issue. It’s a national issue — we just happen to be at the forefront,” Scott said.

Florida agricultural officials immediately announced more aggressive mosquito-control efforts, and Florida politicians rushed to reassure tourists that it’s still safe to visit the state.

Some medical experts said pregnant women should not travel to the Miami area, especially if it involves spending time outdoors. However, the CDC is not issuing such advice.

U.S. health officials said the U.S. might see small clusters of infections. But “we don’t expect widespread transmission in the continental United States,” the CDC’s Frieden said.

The four Florida infections are thought to have occurred in a small area just north of downtown Miami, in the Wynwood arts district, the governor said.

The area, known for bold murals spray-painted across warehouses, art galleries, restaurants and boutiques, is rapidly gentrifying and has a number of construction sites where standing water can collect and serve as a breeding ground for the tropical mosquito that carries Zika.

People in Florida’s Miami-Dade and Broward counties are being tested to learn whether there are more cases, the governor said.

“If I were a pregnant woman right now, I would go on the assumption that there’s mosquito transmission all over the Miami area,” warned Dr. Peter Hotez, a tropical medicine expert at the Baylor College of Medicine.

He said that there are probably more cases that have not been diagnosed, and that people should not be surprised if mosquitoes are soon found to be spreading Zika in Louisiana and Texas as well.

Earlier this week, federal authorities told blood centers in the Miami-Fort Lauderdale areas this to stop collecting blood until they screen it for the virus.

Frieden said the evidence suggests that the mosquito-borne transmission occurred several weeks ago over several city blocks.

Zika primarily spreads through bites from a specific species of tropical mosquito that is found in urban parts of the South and peaks in number in August and September.

So far, there have been than 4,700 cases of mosquito-borne Zika in Puerto Rico and other U.S. territories.

The cycle of infection inside a country can start when a mosquito bites a traveler who has returned home from abroad with the virus still lurking in his or her bloodstream. The mosquito then bites someone else, spreading the virus.

Health officials have been long predicting this would happen in the continental U.S. sometime this summer, probably in Florida and Texas, because of the large numbers of people who travel back and forth to Latin America.

Orange County Mayor Teresa Jacobs, whose jurisdiction includes Walt Disney World and other Orlando-area theme parks, said tourists shouldn’t think twice about coming to the Sunshine State.

Florida had more than 106 million visitors last year, and tourism-related employment accounts for around 1.2 million jobs, making it the state’s biggest industry.

There have been no mosquito-transmitted Zika cases in the Orlando area.

“If you’re coming to Florida as a tourist, if you’re coming to the theme parks, then you’re coming to some of the safest places in the world,” Jacobs said, “because they have mosquito control down like no place else I don’t think in the planet.”

Zika-fighting efforts include pesticide spraying, setting of traps and eliminating standing water around homes.

Florida’s governor has allocated over $25 million in state funds for Zika response, and the White House and the CDC have provided over $10 million.

However, Congress left on a seven-week vacation without giving the Obama administration any of the $1.9 billion it sought to battle Zika.

White House spokesman Eric Schultz called that “regrettable” and said: “Today’s news should be a wake-up call to Congress to get back to work.”

Jenny Gray, who is 27 and works in Wynwood for an art designer, said she will follow experts’ advice to wear insect repellent.

“I don’t plan on having kids now, but I do sometime in the future. Better to stay protected,” she said. “That really does concern me.”

But Phillip Lopez, a 34-year-old Wynwood resident who works at an outdoor bar and exercises outside, said: “It’s a concern, but you got to do what you got to do. You can’t not go outside.”

It’s not unusual that no mosquitoes have tested positive for Zika, said C. Roxanne Connelly, a medical entomology specialist at the University of Florida and a past president of the American Mosquito Control Association.

It can take a couple of weeks before an infected person shows symptoms, and by then the mosquitoes that transmitted the virus are dead, she said.

“Believe it or not, it’s difficult to find positive mosquitoes even when you’re in the middle of an epidemic,” Connelly said. “Sometimes you don’t know where these people were infected. At home? At work? Where they were playing baseball?”

Associated Press writers Mike Schneider in Orlando, Florida; Mike Stobbe in New York; and Josh Lederman in Washington contributed to this report.