Angola is experiencing a major urban yellow fever outbreak that threatens other countries, including China, and is exposing how poorly prepared the world is to fight two mosquito-borne epidemics at once.

Angola’s outbreak, which is known to have killed about 200 people, has already consumed the World Health Organization’s emergency stockpile of yellow fever vaccine.

Image Dr. Margaret Chan of the W.H.O. Credit... Salvatore Di Nolfi/European Pressphoto Agency

The Centers for Disease Control and Prevention has said it cannot give Africa as much help as it normally would: Most of its mosquito-disease experts are fighting the Zika virus in Brazil, Puerto Rico and elsewhere.