Zika virus has been earning all the headlines, because it is already affecting Americans—including 300 pregnant women, according to a new CDC estimate—and is expected to move into U.S. mosquitoes as the summer bug season starts.

But outside the United States, another mosquito-borne disease is attracting the world’s attention, and it may predict more than Zika does about how epidemics will move around the world in the future. The disease is yellow fever, the epicenter of the outbreak is Angola, and the force that could push it around the globe is Chinese investment in the developing world.

The Angolan outbreak began in December and is large: more than 2,400 cases and 298 deaths, according to the latest report from the World Health Organization. It was originally centered on the capital, Luanda, and has spread through the western half of the country. It has also hopped borders: There are 42 cases in the neighboring Democratic Republic of the Congo and two cases in Kenya (along with an an unrelated outbreak in Uganda, between Kenya and the DRC).

But what has some researchers unusually alarmed is that there are 11 cases in China: workers or families who returned from Angola into an area where yellow fever does not now exist—but the mosquitoes that spread it do.

“Approximately two billion people live in Aedes aegypti-infested countries in Asia,” Sean Wasserman, Paul Anantharajah Tambyah, and Poh Lian Lim, researchers from South Africa and Singapore, say in a paper published online in May in the International Journal of Infectious Diseases. “The prospect of a yellow fever introduction into this unvaccinated population poses a major global health threat,” they write.

View Images Maps of Angola showing month-to-month spread of yellow fever. Courtesy the World Health Organization.

Yellow fever is a persistent problem in West Africa, where the virus cycles between monkeys and mosquitoes and spills over to humans. That happens first in villages at forest edges, and then in cities as infected people carry the virus to urban mosquitoes. (These happen to be the same kinds of mosquitoes that transmit Zika, and also chikungunya and dengue: voracious day-biters that breed in pools of water as small as a bottle cap, and attack people not only outdoors but inside houses.)

A vaccine prevents yellow fever, but only about 70 percent of Angolans receive it, not enough to create the herd immunity that would prevent an outbreak from taking hold.

That is a serious gap, because unlike the other diseases carried by Aedes mosquitoes—which except for the birth defects of Zika mostly cause mild illnesses—yellow fever can kill. As many as one in four of those who develop symptoms go on to have liver and kidney failure, jaundice (which gives the disease its name) and bleeding, and one in four of those victims die.

View Images Fumigating a Texas town infected with yellow fever, 1873. Photograph by North Wind Picture Archives, Alamy

Yellow fever has never taken hold in Asia. Lack of familiarity with the disease may explain why the 11 infected people who returned to China were not vaccinated, despite Chinese regulations saying they should have been.

They probably have a lot of company: Angola is one of China’s biggest investment targets in Africa, for cropland and for energy. In 2009, according to the Centre for Chinese Studies in South Africa, China bought almost one-third of Angola’s crude oil. The Chinese expatriate community in Africa is estimated to be 20,000 people, who include not just semi-permanent residents but temporary construction workers who are shipped from job to job.

Because the continents harbor the same mosquito species—Singapore, where Tambyah and Lim work, wages a constant battle against dengue—the researchers suggest that just one unknowing traveler carrying yellow fever virus in their blood could spark a chain of transmission. That could trigger what The Economist warned in an editorial earlier this month would be “a preventable tragedy,” an epidemic as explosive as chikungunya after it arrived in India in 2005, or Zika in the Americas this year.

The authors of the new paper say: “The current scenario of a yellow fever outbreak in Angola, where there is a large community of non-immune foreign nationals, coupled with high volumes of air travel to an environment conducive to transmission in Asia, is unprecedented in history… The growing number of imported cases in China shows how critical it is to recognize this risk now in order to take adequate preventive action so that a global catastrophe can be averted.”

The action that is most needed is vaccination. Monday marked the start of the World Health Assembly, the annual conclave of member states of the World Health Organization. In the opening meeting, director general Dr. Margaret Chan delivered what she called a “stern warning” on failures to vaccinate adequately. (Chan’s term ends in June 2017, so she may have felt safe being blunt—though she did not name names.)

View Images A Rockefeller scientist administers yellow-fever vaccine in Santiago de Guayaquil, Ecuador, in the 1920s. Photograph Courtesy of Rockefeller Foundation

“The world failed to use an excellent preventive tool to its full strategic advantage. For more than a decade, WHO has been warning that changes in demography and land use patterns in Africa have created ideal conditions for explosive outbreaks of urban yellow fever,” she said. “Yellow fever vaccines should be and must be used more widely to protect people living in endemic countries.”

Because of the Angolan outbreak, yellow fever vaccines are in short supply worldwide, as Kai Kupferschmidt reported in Science in April. Only four factories, in Russia, Brazil, France and Senegal, make the compounds, and one is about to close. But in May, a WHO emergency committee declined to rank yellow fever as a “public health emergency of international concern.” As global health-law scholars Daniel Lucey and Lawrence Gostin wrote in JAMA two weeks ago, that designation could have given the agency increased leverage to negotiate with vaccine manufacturers. (Following the decision, the committee advised “rapid evaluation” of dividing vaccine doses so that more people can be protected.)

But that may not be enough. In its editorial, The Economist did the vaccine math: