Teens play at the wharf in Cité Soleil, Haiti, on June 8, 2016. (Bonnie Jo Mount/The Washington Post)

When the Zika virus became a full-blown epidemic this year and global health officials began to anticipate its spread across the Americas, their worst fears settled on the place that looked most vulnerable: Haiti.

The poorest nation in the region appeared to be primed for a Zika explosion, with woeful sanitation, urban overcrowding, a threadbare health system and plenty of mosquitoes.

But nearly six months after the first Zika cases were confirmed in Haiti, the most dire predictions have not materialized. That is the good news. The bad news is that no one is really sure why.

After the World Health Organization declared Zika a “global health emergency” in February, Haiti was tallying 300 infections a week. Now it’s down to about 30.

“The peak seems to have reached Haiti already,” said Jean-Luc Poncelet, the top official for the Pan American Health Organization (PAHO) in Haiti. “But we don’t really know.”

Haiti hasn’t registered a single case of Zika-related microcephaly, the congenital birth defect that results when the virus interferes with fetal brain development. The country has reported a dozen cases of Guillain-Barré syndrome, the sometimes fatal neurological disorder that Zika can trigger.

Across the border in the Dominican Republic, a more prosperous country with a slightly larger population and a significantly better health-care system, authorities have reported 123 cases of Guillain-Barré — more than 10 times as many as in Haiti.

While Zika is thought to be declining in South America, global health officials think the rate of new infection is increasing in the Caribbean. On Friday, the head of the U.S. Centers for Disease Control and Prevention, Tom Frieden, said the virus was spreading so fast in Puerto Rico that “dozens to hundreds of babies” could be born on the island with Zika-related birth defects in the coming year.

Trash burns near a sewage canal in Port-Au-Prince, Haiti, on June 8, 2016. Trash collection is limited and in most places heaps of trash are eventually burned. (Bonnie Jo Mount/The Washington Post)

Haiti’s comparatively low infection numbers do not indicate that it is becoming Zika-free, or even that the outbreak is declining here, experts warn. Rather, they say, Zika’s mysterious trajectory in Haiti may reflect some of the inherent challenges in identifying what happens when the virus fans out across countries with little ability to track it.

Unlike infections such as Ebola, or cholera, or even other mosquito-borne viruses like chikungunya and dengue fever, Zika produces no symptoms in as many as 80 percent of the people who get it. Others typically experience a low-grade fever, skin rash and joint pain that go away after a few days.

In a place like Haiti, that’s not enough for most people to seek medical attention.

“Most Haitians who get a fever don’t go to the doctor. They drink tea,” said Joseph Donald Francois, the national coordinator for cholera and Zika at Haiti’s Public Health Ministry. “I had Zika,” he added, with a shrug.

The virus was expected to take off in Haiti with the onset of summer, when afternoon thunderstorms make pools perfect for breeding mosquitoes in the uncollected garbage that fouls the capital and chokes its gutters.

But aside from a few fliers posted in hospital waiting rooms warning of Zika, there are no signs of a major outbreak. Complicating matters, though, the decline in Haiti’s Zika numbers coincides with a three-month-old strike that has shuttered its public hospitals and left patients in more remote areas with practically no health-care services at all.

Physicians and other health workers march in Port-Au-Prince, Haiti, on June 8, 2016. The workers -- seeking better resources and pay -- have been on strike for months, shutting down public hospitals. (Bonnie Jo Mount/The Washington Post)

A housekeeping crew cleans at Hôpital de l’Université d’Etat d’Haiti in Port-Au-Prince, Haiti, on June 9, 2016. The hospital has been closed for months because of striking health care workers. (Bonnie Jo Mount/The Washington Post)

“It’s hard to know if no information means the absence of disease, or if it means there’s just no information,” said Louise Ivers, an infectious disease specialist with the Boston-based nonprofit Partners in Health, which runs a teaching hospital, considered Haiti’s best, in this small city 30 miles north of Port-au-Prince.

The hospital has had 10 pregnant women diagnosed with Zika so far, and all have given birth to normal babies.

“Zika is here,” said Christophe Milien, an obstetrician-gynecologist in the maternity ward, which delivers about 300 babies a month. “We can’t say Zika isn’t a problem, because there are so many factors that stop women from coming to the hospital.”

Expectant mothers who have other children to care for are not going to seek medical attention when they have a low-grade fever and a rash that will go away after a few days, he said. In more remote rural areas, the nearest highway can be several hours away on foot or donkey. No one has any idea what is happening with Zika in areas like that.

Patients wait for care at Centre Hospitalier Eliazar Germain in Pétion-Ville, Haiti, on June 9, 2016. Demand for care more than doubled at the facility after striking health care workers forced the closure of public hospitals in Port-Au-Prince months ago. (Bonnie Jo Mount/The Washington Post)

Only about one-third of Haitian mothers deliver their babies in hospitals, according to government surveys.

The rest deliver at home, and physicians here concede that babies born with birth defects such as microcephaly are sometimes abandoned in communities where deformities are viewed as the work of evil spirits.

Abortion is illegal but not uncommon in Haiti, and doctors at the Mirebalais hospital say about 10 to 15 women seek medical attention each week after terminating a pregnancy, typically by taking drugs that induce labor and can lead to dangerous hemorrhaging.

Because Zika poses the greatest risk to fetal brain development during the end of the first trimester and beginning of the second, it is possible that the country will begin seeing cases of microcephaly in the coming weeks.

“The fact that we’re not seeing microcephaly suggests the Zika numbers are pretty low, but it just doesn’t make sense,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas. “I predicted Haiti would be hard-hit. So either Zika is being vastly underreported or it’s still in the process of emerging or it’s doing something very different compared to other patterns in the Americas.”

Zika’s other telltale indicator is Guillain-Barré syndrome. The Partners in Health hospital has the only neurologist working in a country of 10 million people, Harvard professor Aaron Berkowitz, who right now splits his time between Boston and Mirebalais, where he is training a Haitian physician.

Berkowitz said he and his Haitian colleagues first noticed something strange in January, when a man came in complaining of facial paralysis. Berkowitz diagnosed him with a rare form of Guillain-Barré syndrome.

“A few days later, another man came in with the same symptoms,” said Berkowitz. The patient went to Boston for treatment, and his blood sample was sent to the CDC for testing, confirming he’d had Zika.

A hospital room at Centre Hospitalier Eliazar Germain in Pétion-Ville, Haiti, on June 9, 2016. Demand for care more than doubled at the facility after striking health care workers forced the closure of public hospitals in Port-Au-Prince months ago. (Bonnie Jo Mount/The Washington Post)

In Brazil and Colombia, which together have reported more than 200,000 Zika infections so far, it’s possible to visit hospitals and find plenty of patients complaining of the virus’s symptoms. Not in Haiti.

At one government hospital in Port-au-Prince that hasn’t been closed by the strike, doctors have not reported a single case of the virus, said medical director C. Henry Gateau. But it has had five cases of Guillain-Barré, one of which was fatal. “We did everything we could,” said Gateau. He added that he had never seen Guillain-Barré in his 15 years at the hospital.

Without the sophisticated laboratory testing that can be done by the CDC, it is very difficult to confirm the presence of Zika, because the virus remains present in the blood for only about five days. But Poncelet, the PAHO director for Haiti, said it would be a mistake to invest in the kind of detailed epidemiological study that could give a more accurate sense of Zika’s spread in the country.

“When you have such limited resources, you can do harm rather than solve problems if you only work on one disease, because it distorts the system,” said Poncelet. He noted that Haiti has 17,000 malaria infections a year and 34,000 cases of cholera.