The Democratic Republic of Congo has confirmed the first case of Ebola in the eastern city of Goma, a major transport hub that is home to more than two million people.
The health ministry confirmed that a pastor tested positive for the virus in a centre in Goma after arriving there by bus on Sunday from Butembo.
The ministry said all others on the bus had been tracked down.
Health workers have long feared the virus could reach the city.
But there was low risk of the disease spreading, the health ministry said.
"Because of the speed with which the patient has been identified and isolated, as well as the identification of all passengers from Butembo, the risk of spreading to the rest of the city of Goma remains low," the statement read.
More than 1,600 people have died since the Ebola outbreak began in eastern DR Congo a year ago - the second biggest outbreak ever.
The pastor travelled 200 km (125 miles) to Goma by bus from Butembo, where he had been with people with Ebola.
The health ministry said in a statement that the bus driver and 18 other passengers would be vaccinated on Monday.
Goma is a major commercial and cultural hub on DR Congo's border with Rwanda, with transport links to the wider region. Its health ministry has been preparing for an outbreak.
In November last year, it launched preparation and response activities. Some 3,000 health workers in the city have already been vaccinated.
But fear of the deadly Ebola virus - which sees patients suffer gruesome symptoms and rules out customary burial rites - is a big challenge for health workers battling to contain the spread.
"People are still afraid to come to health clinics if they are experiencing Ebola symptoms," said the International Rescue Committee's Ebola Emergency Response Director Tariq Riebl.
Decades of conflict in eastern DR Congo have led to widespread mistrust of the authorities and this also has an impact on the disease spreading, according to authors of a recent report.
The current outbreak in eastern DR Congo began in 2018 and is the 10th to hit the country since 1976, when the virus was first discovered.
It is dwarfed by the West African epidemic of 2014-16, which affected 28,616 people mainly in Guinea, Liberia and Sierra Leone. About 11,310 people died in what was the largest outbreak of the virus ever recorded.
Ebola infects humans through close contact with infected animals, including chimpanzees, fruit bats and forest antelope.
It can then spread rapidly, through contact with even small amounts of bodily fluid of those infected - or indirectly through contact with contaminated environments.
Since the beginning of the current Ebola outbreak in eastern DR Congo, the World Health Organization has on three occasions opted not to declare it a global health emergency.
But the UK last week called on the global health body to formally call it an emergency - a technical definition - that would make it easier to raise money internationally.