IT OCCUPIES a strange place on the spectrum of infectious tropical diseases. Not as important as malaria. Not as terrifying as Ebola. Not as revolting as elephantiasis. Yet yellow fever is a grave illness, incurable once contracted. It kills 80,000 Africans a year. And that is a scandal, both because it can be prevented by a single inoculation and also because yellow fever now risks spreading to Asia, where it has never before taken hold.

This is the background to the latest epidemic of the disease, in Angola. Since December, around 2,300 suspected cases have been reported there, with nearly 300 deaths. Set against 80,000 deaths, this may not sound like many. But experience suggests that, for each case brought to the authorities’ attention in a country where health care is as fragmentary as it is in Angola, between 50 and 500 probably go unreported.

Yellow fever is spread by Aedes aegypti, the mosquito that also carries dengue and Zika. Its early symptoms—a high temperature, nausea, vomiting and muscle pain—are reasonably mild and usually last only a few days. In about 15% of cases, however, the disease later returns with a vengeance. Patients experience severe abdominal pain, become jaundiced and bleed internally and from their eyes, mouth and nose. About half of these people die.

The UN and the World Health Organisation (WHO) have shipped 9m doses of vaccine to Angola, enough for about a third of the population. But that is around a fifth of all the vaccine held worldwide at any one time. If the epidemic spreads, stocks will rapidly run out.

And spread it might. Almost 6m people in Luanda, Angola’s capital, should now be immune, and the number of Angolan cases being reported to the WHO has indeed dipped in recent weeks. Yet vaccination rates outside Luanda remain low, and the efforts have not stopped the disease from crossing borders.

Laboratory analyses have linked a few cases in Kenya to the Angolan outbreak. More worrying is the Democratic Republic of Congo (DRC). On May 2nd the WHO reported 453 suspected cases of the disease there, including some in the capital, Kinshasa. Less than 30% of the country’s population was thought to have been vaccinated before today’s outbreak. A booming trade in forged vaccination certificates could also let infected people slip past border checkpoints from Angola into Zambia and Namibia, which reported its first case on April 28th.

The best way to contain the disease now is to vaccinate all those at risk as soon as possible. Every day increases the chance that one of the thousands of Asian workers in Angola will carry the disease home, sparking a full-scale outbreak on a continent that has yet to experience one.

Deployment of the vaccine in all African countries where yellow fever is endemic could slash the number of cases. The Yellow Fever Initiative, which is led by the WHO and UNICEF and funded by GAVI, an international public-private alliance that provides vaccines to poor countries, aims to cover the continent by 2020, at a cost of $300m. More than 100m people have been vaccinated since it started in 2007. With more funding, it might have averted this outbreak: Angola was not among the 12 countries that were considered most susceptible to the disease.

Production of yellow-fever vaccine has increased in the past five years, but it would be difficult to raise further. It has only four sources: Sanofi Pasteur, a French drug company, and institutes in Brazil, Senegal and Russia. “That leaves us in a very vulnerable position,” says Peter Piot, the director of the London School of Hygiene and Tropical Medicine. If yellow fever did take hold in Asia, he says, then the numbers at immediate risk would rise from tens of millions to 100m or more.

The world’s emergency stockpile of 11m doses, which is held on top of normal supply to enable a rapid response to outbreaks, is already being depleted to control the one in Africa. If the disease takes hold in Asia, says William Perea of the WHO, there would be little choice but to limit inoculations to a fifth of a standard dose so as to make supplies of the vaccine stretch further. Small studies give reason to hope that this would protect adults, but the efficacy of a low dose for children is unknown.

Ill winds

International trade and migration mean that the chances of yellow fever spreading to Asia are higher than ever before, warns John Woodall of the Programme for Monitoring Emerging Diseases, an online-alert service. Cool weather has meant that up till now there have been few mosquitoes in China to spread the disease. Even so, the country has already reported its first 11 cases, and summer is approaching. All those diagnosed had returned from Angola, home to an estimated 100,000 Chinese workers.

Once yellow fever is established in a tropical country, it is almost impossible to eradicate. Monkeys infected by the virus act as a reservoir for the disease. People who travel to the jungle can carry it back to towns and cities, where mosquitoes quickly breed—A. aegypti lays its eggs in standing water, meaning that even a discarded food tin could be a breeding ground.

Why Asia has never had a large outbreak of yellow fever is something of a mystery. A. aegypti is found across much of southern Asia (see map), and the continent’s jungles have monkeys that would seem an ideal reservoir for the disease. One possibility is that antibodies against dengue, a related disease, partially protect survivors against yellow fever. A second is that the Asian type of A. aegypti may be less able to carry the virus than its African cousin. But it is not immune. The fear is that a traveller who has returned from Africa with yellow fever will be bitten by an indigenous mosquito, which then spreads the disease.