The Tanzanian health ministry says a mysterious disease that killed five people has been identified as Marburg haemorrhagic fever.
The virus causes high fever, often accompanied by bleeding and organ failure.
It is part of a group of viruses that includes Ebola.
Tanzania's health minister, Ummy Mwalimu urged citizens to stay calm, saying the government had managed to control the spread of the disease.
She said three patients were receiving hospital treatment and 161 contacts were being traced by the authorities.
Last week a rapid response team was sent to the north-western region of Kagera, which borders Uganda, to investigate the outbreak.
The World Health Organization has applauded Tanzania's rapid reaction, adding that it stood ready to ensure "there were no gaps in response".