That turns out to be a lot.
Mosquito control, central to containing the spread, is spotty at best, particularly in impoverished areas with weak tax bases, common in parts of the South. In Tennessee, the overwhelming majority of counties and cities do not have mosquito control programs. In North Carolina, only about a quarter of counties have them.
Dr. Thomas R. Frieden, who heads the C.D.C., said in an interview that although the disease was also transmitted sexually, “mosquitoes are how this is spread,” and the agency is putting significant effort into helping states control the insects. (Mosquito control is a local responsibility, so the C.D.C. will not do the fighting directly.)
What would actually happen, should there be a local case?
The C.D.C. plans to help the local government investigate it and warn residents. The agency detailed how to define the area of transmission — important for warning pregnant women what places to avoid — and underscored the urgency of alerting blood banks. If asked, the agency will dispatch a team of experts to help with everything from logistics to lab testing.
This year, many areas did not even know if they had the mosquito. The C.D.C. updated old maps, but these were pieced together using references from scientific literature and were not meant to be a real-time representation of mosquito range.
With that, states went to work. Mississippi, well within the mosquito zone on the C.D.C. map, started a statewide study of the Aedes aegypti population, testing five areas in every county each month. The result was a surprise: No aegypti.
Dr. Thomas E. Dobbs III, Mississippi’s state epidemiologist, said in an interview that while most counties did not have mosquito control programs, the state had a tiny number of imported cases — three to date, all from Haiti — and considering the fact that the state is so sparsely populated, the risk of transmission was relatively low. (The mosquito flies only about 500 feet in its lifetime, roughly a city block.)
The C.D.C. plan stated that the risk of “prolonged widespread local transmission is not expected,” based on the history of two similar viruses. Of 12 homegrown cases of chikungunya reported in Florida in 2014, for instance, only two appeared to be linked, it said. The other virus, dengue fever, has not spread beyond South Florida and southernmost Texas in the continental United States. Both are mosquito-borne diseases.
And even though most people with Zika have no symptoms, posing the risk of undetected spread, most experts do not believe there will be more than a handful of local cases, mainly because of the conditions of life in the United States — namely, widespread use of air-conditioning and window screens, and relatively little crowding.
A study comparing Laredo, Tex., with its twin just across the border in Mexico — essentially the same city separated by a river — found the incidence of dengue fever was eight times higher on the Mexican side, even though the mosquitoes that carry it were more abundant in Texas. Researchers attributed the Texas advantage to air-conditioning, windows that shut and less crowding within houses.
“Everything we’ve seen from dengue and chikungunya suggests that it will not be a severe problem” in the continental United States, Dr. Frieden said. “Our best guess” is that “we’ll see a singleton case that we won’t be able to identify the source for, and possibly some clusters — maybe in the Florida Keys or Brownsville” in Texas.
Still, he noted that Puerto Rico, an American territory, was facing a public health crisis because of the virus, with potentially “dozens to hundreds of infected infants with microcephaly.”
One of the obstacles for Zika preparedness is money. Congress is still arguing over President Obama’s $1.9 billion request, which was submitted in February.
Dr. Markowski, who spoke by telephone from St. Croix, where he was working on a C.D.C. contract to control mosquitoes in the United States Virgin Islands, said his company had submitted contingency plans to about half a dozen states, including Mississippi, but none have been carried out, possibly because states are waiting for funding — or an outbreak.
Dr. Frieden said longer-term projects were suffering as well, such as “coming up with better diagnostics, coming up with better ways of controlling mosquitoes.” He said the funding holdup has likewise hampered efforts to follow infected pregnant women through their births for multiple years.
Despite the gridlock on funding from Washington, some states, and even cities, are preparing their own plans. Tennessee is doing drills, giving staff members in local health departments surprise scenarios.
“Instead of just letting people tell us theoretically what they think they’d do, we make them prove it,” Dr. Jones said.
He said Tennessee that had set up a Zika response center, but tight funding has meant that the state has had to poach workers from other programs — including H.I.V. and immunizations — to staff it.
“People are enthusiastic about doing it, and it’s the right thing, but it means we’re diverting resources from something else,” Dr. Jones said. “Our surge capacity is not unlimited.”
Dr. Markowski said that he was glad that people were paying attention, but that life should not grind to a halt.
“We shouldn’t live our summer in fear and hide inside,” he said. “We should approach it with the appropriate level of respect that any mosquito-borne disease deserves. But we should also be going outside and enjoying the Fourth of July.”
An earlier version of this article, using information from the C.D.C., erroneously included New Mexico on the list of states where the agency is focusing much of its mosquito control effort. (In addition to the six other states, the C.D.C. is also focusing efforts in Los Angeles County.)