The Zika virus caught the world’s attention last year after experts noticed that a small percentage of women carrying the virus in Latin America were giving birth to children with microcephaly, a defect that causes malformed heads and severely stunts brain development. The virus is now endemic there, and scientists fear that caseloads may spike again during the hot, wet months of the Southern Hemisphere’s summer, which begins in December.

But although the Zika strain circulating in Latin America is originally from Asia, health experts know little about its history on that side of the Pacific. “We know the virus has been circulating in the region for the better part of 70 years, but we don’t know how extensively,” said Duane J. Gubler, an emeritus professor at the Duke-NUS Medical School in Singapore.

Zika may have been understudied for decades because there were no routine tests for it until recently, scientists say, and perhaps also because the symptoms resemble those of mild dengue fever. Another possibility, they say, is that women carrying the Zika virus gave birth to microcephalic children at home. Microcephaly was first noticed in Brazil only because doctors in neonatal intensive care units of several hospitals realized that they had far more children with the problem than they normally did, and many of the mothers reported having Zika symptoms months earlier.

But the Zika scare in Latin America has prompted a recent surge in awareness and monitoring of the virus in Asia. Health ministries across the region have reported hundreds of new infections in recent months. And in October, the World Health Organization said that the Western Pacific region was “highly likely” to report more cases — and possibly new outbreaks.

Q: How does Zika spread?

A: Humans are primarily infected with Zika through the bites of mosquitoes from the Aedes genus — the same ones that carry dengue, yellow fever and other mosquito-borne illnesses. Zika can also be transmitted sexually, although scientists have not yet determined how long the virus lingers in semen or vaginal fluids.

Q: What health risks does Zika pose?

A: The majority of people who contract Zika never experience symptoms, and the symptoms that do appear — including fever, rash, joint pain and conjunctivitis — are often mild.

But Zika can cause microcephaly and other birth defects in infants born to women who are infected during pregnancy. Scientists also believe that Zika infections are linked to the prevalence of Guillain-Barré syndrome, a rare disease of the nervous system that can cause muscle weakness and even paralysis. A Zika vaccine is being developed, but the process may take years.

Q: How quickly is Zika spreading in Asia?

A: In February, as microcephaly and Guillain-Barré cases that scientists believed were linked to Zika appeared in Latin America and the Caribbean, the World Health Organization declared the Zika epidemic a public health emergency. The agency announced an end to that emergency this month but cautioned that the disease should now be viewed, like malaria and yellow fever, as a continuing threat.

By mid-November, 75 countries had reported mosquito-borne Zika transmissions for the first time since 2007, with nearly half of the new transmissions occurring this year alone.

In Southeast Asia, Singapore and Thailand have reported many of the cases this year, with hundreds each. Thailand also reported Southeast Asia’s first two cases of Zika-related microcephaly in infants in late September, and Vietnam reported what it said was probably its first Zika-related microcephaly case in late October.

Q: How concerned should people in Asia be about Zika?

A: The U.S. Centers for Disease Control and Prevention said in a September memo that pregnant women should “consider postponing nonessential” travel to 11 Southeast Asian countries because the virus is linked to birth defects. But the number of reported Zika infections and infant malformation cases with likely links to Zika in the region is still significantly lower than the number in Latin America and the Caribbean. (More than 2,000 babies have been born with microcephaly in Brazil, far more than in any other country.) The CDC has also said that recent reports of new Zika cases in Southeast Asia may simply reflect increased awareness or monitoring, rather than increasing transmission.

Scientists say that tropical Asian countries’ long experience with mosquito-borne diseases may help their response. Girls who contract Zika are believed to be immune when they reach their childbearing years, meaning they cannot give birth to microcephalic babies. People who live in countries that are “hyperendemic” to dengue — including Thailand, Vietnam and the Philippines — also carry natural antibodies that some scientists say may help them to suppress or protect against Zika infections. Dr. Sutee Yoksan, a dengue expert at Mahidol University in Thailand, said that theory is now being tested through research projects in multiple countries. (Other scientists, by contrast, worry that dengue antibodies may make Zika infections worse.)

Southeast Asian officials so far appear relatively sanguine about the risks that Zika poses to their citizens, and dengue remains a greater threat, said Tikki Pang, a former director of policy research at the World Health Organization. But those officials probably would feel political pressure to respond to Zika more aggressively, he added, if it became clearer to them that the virus posed a risk to unborn children.

Q: Are Asia’s health ministries prepared to handle Zika?

A: Asia’s experience with severe acute respiratory syndrome, or SARS, and other infectious diseases has prompted increased coordination among its countries’ health ministries since the early 2000s, health experts said in interviews, and that coordination may come in handy if Zika transmission surges in the region.

But officials may be underestimating Zika’s spread in Southeast Asia, in part because some countries lack the resources to test for Zika infections on a large scale, the experts said. The fight could also be hindered by a chronic lack of political will in the region to address mosquito-borne diseases with long-term investments in vaccine development, public education and new mosquito-control technologies, they said.

© 2016 The New York Times Company