Marburg has caused small, episodic outbreaks since first being identified, most recently last year in Tanzania and Ghana. It is one of the deadliest known viruses, with no approved vaccines or specific treatments.

Marburg virus disease has killed 11 people and sickened 25 others in Rwanda, which declared an outbreak on Sept. 27.

The East African country continues to investigate the source of the outbreak, first traced among patients in health facilities.