Around July 4, a patient entered an emergency room in Miami-Dade County with a fever, a rash and joint pain — three of the four classic symptoms of the Zika virus. By this point, there had already been about 1,600 other Zika cases in the continental United States, but it soon became clear that this one was different.
All the other patients had either traveled to Latin America or the Caribbean, where Zika had been raging for months — or they had sex or close contact with someone who had been there. Not this patient.
It was the case public health officials had been expecting and dreading: A person in the continental United States had been infected from the bite of a local mosquito.
It would turn out to be the first of a wave of cases health officials are now scrambling to identify and contain. They are investigating 17 suspected cases of locally transmitted Zika — including 13 linked to a an area with a radius of 500 feet that touches two neighboring businesses in the Wynwood section of Miami.
While officials are confident that Zika will never run rampant in the United States, the chase is on in South Florida as more local cases are identified and Aedes aegypti mosquitoes, the type that carry Zika, stay one step ahead of the spray.
Public health officials are also grappling with, well, the public: Some think that the authorities should warn pregnant women away from much more than one square mile, and still others seem unaware that Zika, while mild or inconsequential for most people, can cause devastating brain damage to the babies of infected pregnant women.
“Obviously when people detect local transmission, there’s a lot of different opinions,” said Dr. Lyle R. Petersen, who is managing the Zika response of the Centers for Disease Control and Prevention. “People panic and there’s potential for irrational thinking in either direction, not doing enough and doing too much.”
Last week the agency took the unprecedented step of urging people to stay away from a place in the continental United States, warning pregnant women to avoid the square-mile area of Wynwood that contains the patch and the two unidentified businesses.
Dr. Petersen said because the continental United States has better mosquito control, more air-conditioning and less standing water than other countries dealing with the Zika virus, there are most likely to be only “handfuls of local transmission and very rare outbreaks,” which will be containable with a targeted response.
“It’s not the whole city — it’s a very small part of the city,” Dr. Petersen said of the possible risk in Miami. “So the recommendation is just don’t go there, particularly if you’re pregnant. In the rest of the city, you’re more likely to get killed in a car crash than you are to get Zika virus.”
The story of the homegrown Zika cases demonstrates both the value and the limits of planning when the enemy is an unpredictable and stealthy virus delivered by a hardy mosquito adept at hide-and-seek. In a 60-page blueprint this year, the C.D.C. outlined detailed steps to take, and officials have been assiduously tracking the patients, testing people close to them and amping up mosquito control.
Still, battling Zika in Wynwood is challenging because its mixture of businesses, apartments and warehouses makes it a veritable urban mosquito mecca. Slices of the gentrifying neighborhood are bursting with art galleries, boutiques and condominiums, but they give way to a still-tattered section of run-down buildings where residents struggle in poverty.
“This is low-income,” Tony Fonseca, 45, a construction worker, said as he stood outside the La Fama Supermarket at Northwest Second Avenue and 31st Street. “People live on welfare, they use drugs. You walk around here at night, you can get assaulted — they’ll steal your Ray-Bans.”
That part of Wynwood, Mr. Fonseca said, has “lots of standing water,” but he said people in this predominantly Latino neighborhood tend to blame foreign visitors to the arts district for bringing the Zika virus.
“Maybe someone brought it from Latin America,” said Mr. Fonseca, born in Miami of Nicaraguan parents. “But no one around here is worried about it.”
A firefighter taking a break at a coffee shop on Northwest Second Avenue said two of his female colleagues, both pregnant, were temporarily transferred to a station several miles south, near Coconut Grove. But as Florida health department workers go door to door asking for urine samples to test, seeking to learn the extent of the Zika risk in Wynwood, not everyone sees the need.
Diana Ozuna, 27, declined to be tested, even after her 53-year-old mother, who lives nearby, submitted a urine sample of her own. Ms. Ozuna said she lowered her window screens and used repellent, especially on her 20-month-old daughter. Still, because she is not pregnant and has no immediate plans to be, she does not perceive Zika to be a great menace.
“It’s bad,” she said, “but it’s not something that you die from.”
Zika is an enemy most people cannot see. While its effects can be catastrophic to developing fetuses, in adults the effects are usually mild or negligible, and health officials assume that for every person with symptoms, four more have undetected Zika infection.