The World Health Organization director general yesterday openly expressed worry over the “scale and speed” of an outbreak of a rare type of Ebola in eastern Congo, where the authorities reported a sharp increase in suspected deaths — to at least 134 — and more than 500 suspected cases.

The virus spread undetected for weeks after the first known death as the authorities tested for a more common type of Ebola and came up negative, health experts and aid workers said. The Bundibugyo virus has no approved medicines or vaccines.

In Bunia, the site of the first known death, health workers in protective gear moved among residents wearing fabric masks. “I know the consequences of Ebola, I know what it’s like,” said a worried resident, Noëla Lumo.