“A woman could get bitten by a mosquito and have a child with a terrible malformation — and that could happen in Florida or Texas or Arizona, or anywhere this mosquito is,” said Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention. “We don’t think the numbers will be large,” he said, “but the impact could be very large.”

Stopping Zika’s spread, he and other public health experts say, will require vigorous mosquito control across a broad swath of the United States. But the quality of services varies wildly.

Some of the weakest spots for mosquito control are in places where the Aedes aegypti mosquito regularly appears — in Mississippi, Alabama, Georgia and the Carolinas, and along the Texas coast — largely because they lack the tax base to pay for it, said Joseph Conlon, a retired Navy entomologist who is a technical adviser to the American Mosquito Control Association.

If there were ever a place most at risk for homegrown Zika, it is Florida, the state with the largest number of imported Zika cases in the continental United States — 74 reported at last count. And Miami-Dade — the largest port of entry into the United States from the biggest countries in Latin America, according to federal statistics — is the county in Florida with the most reported cases, 32.