A yellow fever vaccination. Reuters

An outbreak of yellow fever that's been affecting the Democratic Republic of Congo and Angola, killing more than 400 people and sickening even more, is poised to spread beyond central Africa. It's already the region's largest outbreak in decades.

Yellow fever is transmitted by the same mosquito that transmits the Zika virus. The symptoms can be anything from a general fever to jaundice (that's where the "yellow" comes in) to a severe liver disease with bleeding.

While the more severe symptoms are relatively rare, the virus kills about half of those who develop them.

There's no treatment once you get yellow fever. There is a vaccine that protects against it, but a current vaccine shortage has some groups worried that there won't be enough doses to contain the most recent outbreak.

"There is no known cure for yellow fever and it could go global," Heather Kerr, country director of the Democratic Republic of Congo for the charity Save the Children, said in a statement.

Saleh Mohammed Hamid, 18, is treated at Teaching Hospital in Darfur after getting yellow fever. Reuters/Albert Gonzalez Farran

On Tuesday, the World Health Organization launched a vaccination campaign with the hope to vaccinate more than 14 million people in affected areas. The organization is working with limited supplies of the vaccine, and it takes a minimum of 6 months to manufacture more vaccines.

To make up for that, the WHO plans to use just a fifth of a vaccine dose per person, which would spread the limited supply to a wider population but protect those people only for about a year.

"We've got to urgently reach as many children and families as we can with the supplies that are left, and this is the only way we are able to do that right now," Kerr said in the statement.

Ten million people at risk of getting yellow fever in Kinshasa, the capital of the DRC. Scientific American's Emily Baumgaertner, reporting from the DRC, found that prevention efforts faced another problem: Hospitals weren't testing patients to see whether they had yellow fever. She reported that it had been more than a month since a yellow fever diagnostic test was run in the country.

The vaccination campaign will have until the start of the rainy season in October to tackle the problem before the mosquitoes that carry the disease breed and infect more people.