The World Health Organization is embarking on a massive vaccination campaign to stem a deadly yellow fever outbreak in Angola and Congo that public health experts say threatens to expand beyond Africa.

The disease has killed more than 400 people and sickened thousands in the two countries, according to an alert that the WHO issued Tuesday.

The organization announced that it needs to vaccinate more than 14 million people in more than 8,000 locations around Congo’s capital, Kinshasa, and along its border with Angola.

The disease, which is transmitted by mosquitoes, is not highly contagious and is preventable through vaccination. Most people who are infected experience mild muscle pain and fever but recover. The virus can, however, lead to organ failure ending in death.

Washington Post