“It also helps that people in the U.S. tend to live fairly far apart in single-family homes,” said Dr. Lyle R. Petersen, who oversees vector-borne diseases at the Centers for Disease Control and Prevention. “This is a mosquito that doesn’t fly very far.”

While the number of new cases of Zika so far is small — last year there were nearly 20,000 infections in Brazil compared to more than 200,000 during the epidemic’s peak — countries like Angola, Thailand, Vietnam and Cape Verde have reported newborns with Zika-related microcephaly, the condition that leaves babies with the misshapen heads and profound neurological damage that stoked global anxiety.

Zika has been taking a path similar to other viral infections that offer immunity to those who have fallen ill and recovered. In Brazil, Colombia, Puerto Rico and other places hard hit by the epidemic, so-called herd immunity may limit the potential for new outbreaks because the virus cannot gain enough traction to spread among those who have never been infected.

But over time, the benefits of herd immunity wane as more children are born, providing fresh tinder for the next epidemiological wildfire. Researchers are also unsure whether those infected with Zika are immune for life, or just for a period of time.

Public health officials have been frustrated by haphazard cooperation from countries worried about the stigma associated with Zika as well as those overwhelmed by other health crises. In Angola, the government did not initially report dozens of microcephaly cases that were first discovered by Portuguese researchers. Earlier this year, India protested its inclusion on the C.D.C.’s advisory list for pregnant women during a Zika outbreak in the country’s northwest. In April, the C.D.C. modified its warning.

Dr. Eve Lackritz, who leads W.H.O.’s Zika Task Force, said one of her main tasks is to keep up the sense of urgency. “My biggest fear is complacency and lack of interest by the global community,” she said.