Almost 80 years after the yellow fever vaccine was created in a New York laboratory, a massive outbreak of the disease has killed hundreds of people in Angola, where most were never immunised.
Now, the virus is jumping across borders into other nations whose populations are also largely unvaccinated.
More than 3000 suspected cases are in Angola and 1000 in neighbouring Congo, making this the biggest urban epidemic in decades. More than 400 people have died. There are growing concerns that Chinese workers - of whom there are thousands in Angola - will carry the virus to Asia, where nearly all of the rural poor are also unvaccinated.
The explosion of yellow fever has put severe strain on stockpiles of the vaccine. And the four major manufacturers that produce the vaccine cannot make enough to conduct the kind of campaign that would quickly halt the spread of the disease in other parts of the region.
Yellow fever was once a devastating scourge in the West - in 1702, New York City lost 10 percent of its population to the virus. Thanks to the vaccine and mosquito eradication programmes, it faded in the United States long ago. The fact that the disease is emerging again as an international threat reflects a lack of preparedness by local and global health institutions, and Africa's transformation into a more urbanised and interconnected continent.
Fourteen years after the end of a brutal civil war, Angola boasts road networks and airlines that allow more people to travel at a faster pace than ever before. Yellow fever - which in recent decades has emerged again in remote, sparsely populated locations - has taken advantage of the same infrastructure to spread.
For years, the World Health Organisation kept six million doses of the vaccine in case of a public health emergency. But, as is apparent, that was not nearly enough for a fast-moving epidemic that crosses porous borders. In Angola, even after a campaign that vaccinated six million people in Luanda, millions of people remain unvaccinated. If swaths of rural China had to be immunised, the demand for vaccines would skyrocket.
The factories that make the vaccine "are above the capacity of production. They don't produce more than 2.4 million doses per month. That hampers what we are doing. We need to vaccinate 25 million people" in Angola, said Hernando Agudelo, the WHO representative in this former Portuguese colony.
Yellow fever is far less deadly than Ebola, or even malaria, and it is much easier to safeguard people from the disease. One jab of the vaccine serves as a decade-long inoculation. But each dose takes about 12 months to make - hindering a nimble response to an expansive epidemic.
"It is tragic, because we have an intervention that makes the disease entirely preventable," said Ray Arthur, the director of the Global Disease Detection Operations Centre at the US Centres for Disease Control and Prevention.
In Luanda, the Angolan capital, the disease haunts some of the city's most desperate communities. Outside of the central hospital one recent morning, a woman whose younger brother had just died of yellow fever wailed uncontrollably. A few kilometres away, at Hospital dos Cajueiros, several patients - feverish and delirious - said they were sure they had the illness.
At the sprawling Camama cemetery, where a stream of mourners affix pictures of the dead to their cars, a man looked at the picture of a baby girl in a pink dress named Marisa Jose Almeda taped to a pickup truck.
"Febre amarela," he said, Portuguese for yellow fever.
Small outbreaks of yellow fever are common in remote parts of Africa and Latin America. But urban epidemics are particularly frightening since the disease can spread rapidly in heavily populated areas where many residents are not vaccinated.
In the past, yellow fever outbreaks "happened in the forest in general. It happened in tropical areas where there are bushes and so on," Agudelo said.
But, he added: "The border between the rural areas and the urban areas is disappearing."
Now, Angolans can get on a bus or a plane in the jungle and, within hours, land in the sprawling capital, where the mosquitoes that serve as carriers of the disease swarm, especially during the rainy months. They are even more ubiquitous this year, since an economic downturn and significant budget cuts led to a reduction in sanitation services and garbage collection.
Since January, there have been more than 3000 cases of yellow fever reported in Angola and probably many more unreported ones. About 350 people in the country have died of the disease.
At first, the disease can look a lot like malaria: Symptoms include fever, muscle aches and nausea. But yellow fever often turns the skin sallow and causes victims to bleed from the nose, mouth and eyes. About 5 to 10 per cent of people who contract the disease die, and most of those within 10 days.