Angola's ailing national health system has taken a double blow with a yellow fever outbreak and a fall in funds for overcrowded hospitals and the battle against the world's highest infant mortality rate.

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.

NICHOLE SOBECKI FOR WASHINGTON POST Women grieve over the casket of a young boy who had died two days prior in Luanda, Angola. The family said doctors had been uncertain whether the cause of death was yellow fever or malaria.

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.

THE WASHINGTON POST More than 3000 suspected cases are in Angola and 1000 in neighbouring Congo, making this the biggest urban epidemic in decades.

Yellow fever was once a devastating scourge in the West - in 1702, New York City lost 10 per cent 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 6 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 6 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 Center at the US Centers 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 miles 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.

In 2006, the WHO and UNICEF had begun immunising high-risk countries through the Yellow Fever Initiative, a programme supported by governments, nonprofit groups and vaccine manufacturers.

But because the supply of vaccines was limited and the capacity to conduct large public-health campaigns was weak in many sub-Saharan African countries, the scale of the initiative was restricted.

Countries such as Nigeria and Sudan were prioritised, since they had more recorded outbreaks of the disease.

Given the lack of resources, said William Perea, coordinator for the Control of Epidemic Diseases Unit at the WHO, "you cannot vaccinate everywhere at the same time. You have to make some prioritisation."

During outbreaks in Africa in the 1990s and 2000s, the disease was mostly concentrated in small rural villages or forested communities.

But after Angola's 27-year civil war ended in 2002, a development boom transformed Luanda and nearby areas.

The country quickly became Africa's second-biggest oil producer, creating an economic boost so big that it drew thousands of migrant workers from other nations.

Just as the spread of Ebola in 2014-2015 highlighted the growing urbanisation and the relative ease with which people can move between West African countries, cities and villages, yellow fever is underscoring the changes in this part of southwest Africa.

The first recorded cases here involved two Eritrean migrants who fell ill in the market town of Viana, on the outskirts of the capital.

They had vaccination papers that later turned out to be fake.

In some countries, it is cheaper or easier to buy fake vaccination papers than to pay for the shot.

The Angolan government responded by launching a major vaccination campaign in Luanda, the epicentre of the outbreak. But by the time it began, the disease had spread across the country.

"As we vaccinated one area, it moved to another area," said Amanda McClelland, an emergency health expert at the International Federation of Red Cross and Red Crescent Societies.

"One of the challenges with this response is that we've been chasing the virus with these immunisation campaigns," said Arthur, the CDC expert.

Now, Angolan and international health officials say they would like to vaccinate the country's entire population.

But the vaccine shortage complicates that plan.

Earlier this month, the WHO said it could potentially reduce the vaccine dose to one-fifth of its normal strength, and that would still immunise people for at least 12 months.

"More research is needed to find out whether fractional doses would be effective in young children, who may have a weaker immune response to yellow fever vaccine," the WHO said in a statement.

A yellow fever epidemic might seem anachronistic to people in the United States and Europe, where the disease no longer poses a threat.

But some of the most devastating urban outbreaks of yellow fever have occurred in America. In the 18th century, the disease was called the "American Plague."

In 1793, it killed thousands in Philadelphia as the city panicked. In 1878, it killed 13,000 in the Mississippi Valley.

"It is impossible to estimate with any approach to accuracy the loss to the country occasioned by this epidemic," President Rutherford B. Hayes said in a speech to Congress after the 1878 outbreak.

Those epidemics led to a range of public health reforms and, eventually, the creation of a vaccine.

More than a century later, experts say there are lessons to glean from the Angolan outbreak.

"Given what happened in Angola, we need to concentrate a lot more on major urban areas," said Perea, the WHO expert.

"Many of these cities were connected very poorly to the global network. Now, they are extremely well connected. Ten years ago, you didn't have that."