For much of the past year, the Zika virus has dominated the news cycle and commanded international attention. With the Summer Olympics in Rio de Janeiro less than two weeks away, and two cases of the virus that could be unrelated to travel reported in Florida, that is unlikely to change any time soon. But another mosquito-borne disease, yellow fever, is working its way (albeit more quietly) through the African nations of Angola and the Democratic Republic of the Congo. Though it has attracted far less attention than the Zika virus, yellow fever nevertheless could disrupt economic activities in and beyond those countries, a key consideration in assessing the geopolitical risk of a disease outbreak. What's more, a large number of foreign workers in Angola and the Congo as well as a vaccine shortage worldwide could conspire to turn the latest outbreak of yellow fever -- a disease that has...