The Marburg virus has killed five people in Tanzania’s northwestern Kagera region, the health ministry said.
High fever is a common symptom of deadly viruses like Ebola, which is often followed by bleeding and organ failure.
Tanzania’s health minister Ummy Mwalimu said the disease was contained and she was confident it would not spread further.
Three people are being treated in hospital and authorities are tracing 161 contacts, Ms Mwalimu added.
The World Health Organization (WHO) praised Tanzania’s approach to control the spread.
WHO’s regional director for Africa, Dr Matshidiso Moeti, said: “The efforts of the Tanzanian health authorities to establish the cause of the disease is a clear indication of the determination to respond effectively to the outbreak.”
Dr Moeti said the WHO was working with the Tanzanian government to “rapidly scale up control measures to stop the spread of the virus”.
The Marburg virus is a cousin of the equally deadly Ebola virus – part of the filovirus family – and it kills on average half of those infected, the WHO said.
It is a serious, often fatal disease with symptoms including headache, fever, muscle aches, vomiting blood and bleeding.
No vaccines or anti-viral treatments have been approved to treat the virus, the WHO said, but it added that rehydration with oral or intravenous fluids improved survival.
Recent Marburg infections and deaths are centered on the African continent, where hundreds of people have died from the virus in the past.
The virus was first identified in 1967, after 31 people were infected and died simultaneously in Germany and Serbia.
Since then, there have been outbreaks in Guinea, Uganda, Angola, the Democratic Republic of Congo, Kenya and South Africa, the WHO reports.
Equatorial Guinea reported its first outbreak last month. According to the WHO and local authorities, at least nine people have died.
Last July, two people in Ghana died from the virus and 98 contacts were quarantined. Two months later, the country declared the end of its outbreak.
In 2021, Guinean health officials confirmed the first case of the virus in West Africa, while an outbreak in Angola in 2005 killed more than 300 people.
Egyptian rousette fruit bats usually carry the virus, but African green monkeys and pigs can also carry it.
In humans, it is spread through bodily fluids and contact with contaminated bedding.
Additional reporting by Rhoda Odhiambo and Alfred Lasteck.