Two tourists in Miami share their thoughts about their risks from the Zika virus and the precautions they are taking.  Federal health officials on Monday urged pregnant women to stay away from a Miami neighborhood where they have discovered additional cases of Zika infection — apparently the first time the Centers for Disease Control and Prevention has advised people not to travel to a place in the continental United States.  Florida officials said the number of Zika cases caused by local mosquitoes had risen to 14 from the four announced on Friday: 12 men and two women. They declined to say whether either woman was pregnant. All of the cases have been in one neighborhood.  Health officials said they still did not expect the number of local cases to grow into anything comparable to the epidemic that has raged across Latin America in recent months.  The 10 newly identified patients were most likely infected weeks ago, as early as mid-June, the officials said.  But the new information casts doubt over the effectiveness of weeks of intensive mosquito-control efforts in South Florida and raises questions about tourism in the state, which drew more than 100 million visitors last year.  Dr. Thomas R. Frieden, the director of the C.D.C., said that the Aedes aegypti mosquito, which transmits the Zika virus, has proved to be a wily adversary in Wynwood, a crowded, urban neighborhood in north Miami where all the cases were found. The mosquito may be resistant to the insecticides being used or may be able to hide in standing water.  “Aggressive mosquito control measures don’t seem to be working as well as we would like,” he said in a press briefing on Monday.  The authorities had expected additional cases of Zika infection linked to the neighborhood, he said. But officials were particularly concerned by indications over the weekend that “moderately high” numbers of Aedes aegypti mosquitoes and their larvae were still being found in a one-square-mile section in Wynwood, an area of warehouses, art galleries, restaurants, bars, apartments and condominiums.  “We advise pregnant women to avoid travel to this area,” Dr. Frieden said, “and pregnant women who live and work in this area and their partners to make every effort to avoid mosquito bites and practice safe sex.”  Why scientists are worried about the growing epidemic and its effects on pregnant women, and how to avoid the infection.  Pregnant women who traveled to the neighborhood on or after June 15 should talk with their doctors about getting tested for possible infection, he said.  In addition, said Dr. Denise J. Jamieson, a leader of the C.D.C.’s pregnancy and birth defects team, “we are recommending women who are considering pregnancy not get pregnant for up to eight weeks after returning from that area.”  No mosquito found in the neighborhood has tested positive for the virus, but this species has a short life span. Health officials said the Florida mosquitoes carrying the virus had probably acquired it by biting an infected traveler from Latin America or the Caribbean.  At the request of the state, the C.D.C. has dispatched an emergency response team of eight experts in fields including birth defects and mosquito control.  Dr. Frieden said mosquitoes in the neighborhood would be tested for resistance to the insecticides being used, which he said were two different types of pyrethroids.  In a sign that the Zika cases might affect tourism in Florida, Britain’s health agency, saying the risk was moderate, advised pregnant women on Saturday to “consider postponing nonessential travel to affected areas until after the pregnancy.”  Jack Ezon, the president of Ovation Vacations in New York, said that his agency received 22 reservation cancellations on Monday for trips to Florida over the next six months, and that about four times as many people called for information about the travel advisory.  “Yesterday, the news was terrorism. Today, the news is Zika,” he said.  The airline JetBlue said in a statement that it would allow refunds for people with “concerns of traveling to Zika-impacted areas confirmed by the C.D.C.”  But Ross Feinstein, a spokesman for American Airlines, which has a hub in Miami, said no refunds would be issued, even for pregnant women. “The C.D.C. advisory doesn’t tell people they shouldn’t travel to Miami,” he said.  Abraham Pizam, the dean of the Rosen College of Hospitality Management, said that if the travel advisory had any effect on tourism, it would be only “for a very short period of time” and would not put a big dent in the diverse range of tourists who visit Florida.  Mapping where Zika infections have been reported.  Gov. Rick Scott said in a statement that the Florida health authorities had tested people in three locations in Miami-Dade and Broward Counties and ruled out transmission by local mosquitoes in two of those locations. Six of the 10 newly identified cases were asymptomatic and discovered through the door-to-door testing.  Dr. Frieden said 12 of the 14 cases were linked to a 150-meter area around two workplaces where the initial two cases were identified in July.  Since July 7, when Florida began investigating what turned out to be the first diagnosed local transmission, more than 200 people in Miami-Dade and Broward Counties who live or work near the Wynwood neighborhood have been tested, Mr. Scott said.  Mosquito experts said Aedes aegypti thrives in tiny pockets of water — a bottle cap, a storm drain — making for a notoriously elusive foe.  “Very little historically has worked on Aedes aegypti, owing to its stealthy and domestic behavior,” said Phil Lounibos, a professor of entomology at the University of Florida.  Aerial spraying is largely ineffective in an urban setting like Wynwood. Instead, mosquito control employees have conducted 621 property inspections there, treated 21 properties for mosquito breeding and used portable sprayers against adult mosquitoes on 100 properties, said Gayle Love, the spokeswoman for the Miami-Dade County Department of Solid Waste.  Spray trucks were also sent to the neighborhood, but were legally allowed to spray only at night, while Aedes aegypti mosquitoes tend to bite during the day, said Michael S. Doyle, the executive director of the Florida Keys Mosquito Control District.  “Most of the chemicals that are available for mosquito control are not effective against Aedes aegypti — they are resistant,” he said.  On Monday, in Wynwood, there was little obvious concern.  “I haven’t seen any panic,” said Virgil Cantú, 22, a bartender at the Bar Next Door.  Melanie Hernandez, 20, who was working Monday at the Marine Layer clothing store in Wynwood, said she was worried, even though she did not plan to get pregnant anytime soon. “I looked up the symptoms online after I heard that Wynwood is ground zero for Zika,” she said.  “Obviously you’re scared, because you never know.”  Lizette Alvarez and Nick Madigan contributed reporting from Miami, and Karen Schwartz from Fort Collins, Colo.  A version of this article appears in print on August 2, 2016, on page A1 of the New York edition with the headline: A Travel Warning for Miami as Zika Cases Rise. Order Reprints| Today's Paper|Subscribe
At least 14 cases of the virus been found in the Wynwood neighborhood in Miami and have been spread by local mosquitoes there.