A National Institutes of Health official says the Zika virus could "hang around" the United States for a year or two.
Dr. Anthony Fauci told ABC's "This Week" on Sunday that other Gulf Coast states, besides Florida, are most vulnerable to the spread of the disease.
Mosquito-borne Zika cases have been found in two neighborhoods of Miami-Dade County. They are the first areas on the U.S. mainland where health officials determined mosquitoes were transmitting Zika.
Meanwhile the mayor of Miami Beach -- where five mosquito-borne cases have been found -- says city workers are doing everything in their power to go after mosquitoes in the popular tourist destination.
Mayor Philip Levine told New York radio station AM 970 that Miami Beach is running smoothly, despite the Zika concerns.
Miami-Dade County officials and business leaders are confident South Florida will retain its status as a world-class tourist destination.
Levine said during a news conference Friday afternoon that the Zika reports certainly aren't ideal for tourism, but he expects the long-term impact to be relatively minor. He said city workers are trying to get rid of standing water and foliage that might attract the virus-spreading insects, while the county begins a fumigation program to kill the bugs.
"Between our efforts and the county's spraying efforts, the last thing I'd ever want to be on Miami Beach is a mosquito," Levine said.
The director of the U.S. Centers for Disease Control and Prevention says officials will not be able to spray the South Beach area to prevent Zika as they have in other areas because of high-rise buildings and strong winds.
Miami-Dade County officials have been spraying pesticide from planes flying over Miami's Wynwood arts district since early this month, when Zika transmissions by mosquitoes were confirmed there.
Dr. Tom Frieden says planes can't fly low enough among Miami Beach's high-rises to spray pesticides that kill mosquitoes and their larvae. He says strong winds over the narrow island city also hinder such flights.
Frieden also says the large numbers of residents and visitors in the area will make it challenging to control the spread of Zika there.
The top Democrat in the Senate says Congress must return from its weekslong summer break to deal with the Zika virus, an unlikely scenario in light of the dysfunction that prevented lawmakers from agreeing on money to fight the mosquito-borne disease.
Sen. Harry Reid of Nevada said in a statement Friday that the transmission of Zika in Miami Beach was the most alarming development yet, and the threat to the country's health and economy has never been greater. He said the American people cannot afford to wait any longer for action.
President Barack Obama requested $1.9 billion in emergency funds in February to develop a vaccine and control the mosquitoes that carry the virus. But lawmakers left Washington in mid-July for a seven-week recess without approving any of the money.
Abortion politics played a central role in the impasse.
Republicans angered Democrats by adding a provision to a $1.1 billion take-it-or-leave-it measure