The World Health Organization’s leader traveled to the epicenter of the Ebola outbreak in the Democratic Republic of the Congo over the weekend, as suspected cases and deaths continue to mount.

Tedros Adhanom Ghebreyesus, WHO’s director-general, visited Congo’s eastern province of Ituri, and on Monday was expected to meet with the country’s president.

More than 1,100 cases are suspected in what has quickly become the third-largest Ebola outbreak since the virus was discovered half a century ago. More than 350 deaths are suspected.

No cases have been confirmed outside of Congo and neighboring Uganda. Ebola tests came back negative on Sunday and Monday for two patients in Brazil — one in Rio de Janeiro and the other in São Paulo — who doctors had suspected might have Ebola.

Echoing the fears of many global health experts and doctors, the International Rescue Committee, a global humanitarian organization, warned Monday that the outbreak is likely significantly larger and more advanced than official figures suggest because of delayed detection and poor contact tracing.

“With only 20% of contacts currently being traced, health authorities are struggling to identify and isolate new chains of transmission,” it said in a statement.

After visiting a newly opened Ebola treatment facility in Bunia on Sunday, Tedros emphasized that early medical intervention can save lives.

“Even without vaccines or specific therapeutics, people can survive Ebola disease caused by the Bundibugyo virus if they receive timely healthcare and seek treatment as soon as symptoms appear,” he said Monday on X.

On Sunday, WHO announced that four nurses had been discharged from a hospital in Bunia after recovering from Ebola. A laboratory worker had also recovered earlier this week, the agency said, bringing the confirmed number of people who have recovered from the virus in Congo to five.

Dr. Jean Kaseya, director-general of the Africa Centers for Disease Control and Prevention, wrote in an op-ed in the Financial Times on Sunday that Congo and neighboring Uganda had confirmed 263 cases and 43 deaths. WHO reported the same number of confirmed deaths Sunday but said there were 291 confirmed cases between Congo and Uganda.

“We must move at the speed of the epidemic,” Kaseya wrote in the op-ed, adding that “the risk of regional spread is already happening.”

Health officials and medical workers face “persistent challenges” in containing the outbreak, a joint statement issued by the Congo government and WHO said, including early detection and isolation of cases, contact tracing, and safe and dignified burials of the victims.

Last month, WHO declared the outbreak in Congo and Uganda — caused by the rare Bundibugyo version of the virus — a public health emergency of international concern.

The outbreak is outpacing the global response, as doctors in the region play catch-up. Fear and anger over the health crisis among local communities have at times turned violent.

“Never before has an Ebola outbreak recorded so many cases so soon after its declaration,” Doctors Without Borders (Médecins Sans Frontières), a charity that provides humanitarian medical care, said in a statement Saturday. It also called for more medical staff and testing on the ground: “Like everyone in the affected areas, MSF teams are witnessing a response that has not yet caught up to the rapid spread of the epidemic.”

Health authorities thousands of miles away are on high alert.

In São Paulo, Brazil, the health department ruled out Ebola in a man who had recently traveled to Congo and had tested positive for meningitis. And in Rio de Janeiro, a traveler from Uganda who had contracted malaria tested negative for Ebola via saliva and urine tests on Sunday. Brazil’s Ministry of Health said the patient’s blood sample is still being analyzed, but the chances of a positive result are low.

In Italy, protocols for a suspected case of Ebola were triggered in Sardinia’s capital, Cagliari, after a man who had flown back from Congo on Saturday presented some symptoms. But the Health Ministry said Monday that the patient had tested negative.