Yellow fever is a preventable killer without a cure. Unlike for Zika (or Ebola, and other re-emerging threats), there already exists a yellow fever vaccine—one that's cheap, lifelong, and highly efficacious. Yet southern Africa's worst yellow fever outbreak in three decades—which has infected almost 5000 and killed 428, without slowing—is demonstrating its potential to annihilate when left in the shadows.
The outbreak arose in the midst of the perfect storm. First, in the fall of 2015, dropping oil prices for Africa's second largest crude exporter, Angola, ushered in cuts in public sanitation services; in turn, urban trash build-up created an optimal breeding ground for the Aedes aegypti mosquitoes that carry yellow fever. Second, it arose at the onset of the rainy season, which meant more stagnant water in old tires and stray bottle caps, where mosquitoes breed. And most of all, it arose while the world was turned the other way, frenzied by the vogue new epidemic, Zika. Zika and yellow fever are closely related: they're carried by the same mosquito and spread the same way. Yet while Zika has harmful long-term effects on adults and fetuses, its cousin, yellow fever, kills.
When the outbreak sprang up in Angola, an immediate capital-wide campaign exhausted the entire global supply of yellow fever vaccines. Now, it's spreading wildly through the unvaccinated Democratic Republic of the Congo, with exported cases in several African nations and even China. The stakes are enormous: Chinese investment in the region means that the handful of ill Chinese workers' return to Aedes aegypti-infested Asia could spark transmission among 100 million people living in large cities, or if it reaches the jungle, among one billion on the globe's most populous continent.