GENEVA (AP) — The World Health Organization said Saturday it still doesn't know what caused the death of a young worker at a lab in Russia that focuses on plague and other illnesses.

The U.N. health agency has cited initial, unconfirmed reports suggesting pneumonic plague, a severe disease less common than the better-known bubonic plague linked to the Black Death in the Middle Ages. Pneumonic plague spreads from person to person, causing a severe form of pneumonia, but recovery r...