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About 98 percent of 2,694 documented contacts — people exposed to Ebola by a confirmed patient — are being monitored, Ogwell said, adding that it “gives comfort that we know the evolution of this particular outbreak.�

Tracing contacts is key to stemming the spread of contagious diseases like Ebola.

Uganda declared an outbreak of the Sudan strain of Ebola on Sept. 20. The epicenter is a rural community in central Uganda about 150 kilometers (93 miles) from the capital, Kampala.

Health officials in the district of Mubende, where the outbreak began, were not quick to confirm Ebola, partly because the disease’s symptoms can mimic those of the more prevalent malaria. A lockdown has since been imposed there and in a neighboring district as part of efforts to stop the spread of Ebola.

But the outbreak is spreading in Kampala after some Ebola patients traveled to seek treatment there. Six schoolchildren attending three different schools are among at least 15 confirmed Ebola cases in Kampala, the health minister reported Wednesday, raising fears of contagion.

At least 170 contacts of the six schoolchildren are being monitored, Health Minister Jane Ruth Aceng said.

Administrators at two of the three Ebola-affected schools told the AP Thursday they would not shut down completely. But they suspended learning for those classes attended by Ebola-sickened children, one of whom has since died.

Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, expressed concern over rising cases in new geographical areas in a series of tweets late Wednesday. “Ebola control in urban areas can be complex & requires coordinated & sustained efforts to interrupt transmission,� he said.

There is no approved vaccine for the Sudan strain of Ebola, but two candidate vaccines are due to be tested in clinical trials that officials say will launch in days.

The current outbreak of Ebola, which manifests as a viral hemorrhagic fever, has so far infected 109 people and killed 31, including four health workers, according to the Africa CDC.

Ebola is spread by contact with bodily fluids of an infected person or contaminated materials. Symptoms include fever, vomiting, diarrhea, muscle pain and, at times, internal and external bleeding.

Uganda has had multiple Ebola outbreaks, including one in 2000 that killed more than 200 people. The 2014-16 Ebola outbreak in West Africa killed more than 11,000 people, the disease’s largest death toll.

Ebola was discovered in 1976 in two simultaneous outbreaks in South Sudan and Congo, where it occurred in a village near the Ebola River, after which the disease is named.