A suspected pneumonic plague has killed 100 people in eastern Democratic Republic of Congo, according to preliminary test results.
Plague is passed to humans through flea bites
The World Health Organization (WHO) said isolation wards had been set up to treat patients.
But it says control measures have been hard to implement because of security concerns in the area.
Nineteen of the reported deaths were in DR Congo's Ituri province - the area worst-affected by plague in the world.
The outbreak began there in mid-May.
Suspected cases of bubonic plague have also been reported, but the total number is not known.
A team from Doctors without Borders, the WHO and the Congolese Health Ministry are providing support to local health authorities.
Both strains of plague are spread mostly by fleas, causing an infection in the lungs which slowly suffocates the victim.
If treated promptly with antibiotics, the victim generally survives.
When left untreated, the pneumonic strain - which can also be spread from human to human via respiratory droplets - has a very high fatality rate.