Controlling the Zika virus in the United States just got harder. The mosquito- and sexually-transmitted disease has now likely gone local, according to federal public health officials. In at least four instances patients have apparently contracted the virus via a bite from a mosquito in the continental U.S.

This first recorded instances of local transmission—reported in Florida—signal a shift in the burden of Zika in the mainland U.S., where more than 1,600 people have been diagnosed with it after traveling elsewhere in the Americas or the Caribbean and returning with the virus in their systems. As of July 27 another 15 had acquired the virus via sexual contact with a person who was infected with the virus outside of the mainland U.S.

Total cases in U.S. states: 1672 (14 locally acquired mosquito-borne cases, 1,657 travel related, 1 laboratory acquired); Total cases in U.S. territories: 4,750 (4,729 locally acquired, 21 travel related). Data shown as of July 27 according to the CDC, updated with 14 apparent locally acquired cases total reported by the Florida governor on July August 1.

Graphic by Amanda Montañez

The long-anticipated incidences of local transmission are not a surprise. For months Tom Frieden, the director of the U.S. Centers for Disease Control and Prevention, and other top administration officials have said that they expected the virus to crop up in the mainland U.S. in small outbreaks, particularly along the Gulf Coast where mosquito-borne dengue and chikungunya—carried by the same species of mosquitoes that likely carry Zika—have been documented before. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, has repeatedly said to expect dozens or scores of locally-transmitted cases. Most officials believe a large-scale outbreak remains unlikely. Yet whether outbreaks may include one part of the country, a city or part of a city is difficult to predict.

Florida Gov. Rick Scott said at a news conference Friday that three men and one woman in Miami-Dade and Broward counties had likely contracted the virus via local mosquito bites, after officials could not explain the infection in any other way. The patients had not recently traveled to a place where Zika is actively spreading, nor had they been in close contact with someone who had been traveling in such areas.

“All the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago in several blocks in Miami,” Frieden said in a CDC news release. “We continue to recommend that everyone in areas where Aedes aegypti mosquitoes are present—and especially pregnant women—take steps to avoid mosquito bites. We will continue to support Florida’s efforts to investigate and respond to Zika and will reassess the situation and our recommendations on a daily basis.”

The two mosquito species typically responsible for spreading the virus are present in more than two-thirds of the U.S. Their range extends from the southern part of the country into parts of the Midwest and the Northeast. But many factors have to align in order for the mosquitoes to pass on the virus. First, a female mosquito (males do not bite) must feed on a human carrying the virus. Next the virus must incubate in the mosquito’s body for about a week, and only then can it be transmitted if the insect bites another human. The adult lifespan of a mosquito is typically only a few weeks, so the virus has a better chance of spreading if its host can feed in a densely packed area—like a city. Conveniently for the virus, these mosquitoes like to live alongside humans and are frequently found in the home or breeding in small pools of standing water in people’s yards.

Zika symptoms are usually mild and may include rash or fever, although most patients do not even feel sick. The Zika response has instead focused on protecting pregnant women from contracting the virus because it can lead to birth defects including microcephaly, a condition where babies are born with abnormally small heads. (The World Health Organization has also said that there is “scientific consensus” that the virus can cause the sometimes-paralyzing autoimmune disease Guillain-Barre Syndrome in patients of any age.) To help control local Zika spread, public health officials have reiterated calls for people to eliminate standing water from around their homes and to try to avoid mosquito bites by wearing long sleeves, long pants and approved bug sprays.

“We anticipate that there may be additional cases of ‘homegrown’ Zika in the coming weeks,” Lyle Petersen, incident manager for CDC’s Zika virus response, said in the news release. “Our top priority is to protect pregnant women from the potentially devastating harm caused by Zika.”

The CDC and health officials have already gained some experience by trying to control the spread of Zika in U.S. territories. To date, Puerto Rico has represented the frontlines of the U.S. battle with more than 4,600 cases of the virus locally transmitted by mosquitoes. Public health officials there have ramped up their mosquito control efforts and their messaging urging people to protect themselves against bites. But they have faced myriad obstacles, including mosquitoes’ resistance to certain common insecticides and the fact that many buildings do not have screens in their windows. The heavy burden of controlling Zika prompted the U.S. territory to begin importing all of the blood it might need for medical procedures from the mainland in March, and to freeze prices for bug spray and condoms to stave off Zika-related price gouging. “Nothing about Zika is going to be easy or quick,” Frieden said in April. “The control of this particular mosquito is hard and although we are learning a lot quickly there is still a lot we don’t know.”

*Editor's Note (8/1/16): Story updated at 11:45 AM to reflect new local Zika numbers in Florida.