The United States on Monday warned its citizens that they should reconsider traveling to Rwanda, citing an outbreak of the deadly Marburg virus.

Amid an outbreak of Marburg virus in Rwanda, travelers coming into the United States who have been in Rwanda in the previous 21 days will be screened starting next week, the US Department of Health and Human Services announced Monday.

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.