KAMPALA, Uganda — Ugandan officials have reported 11 more cases of Ebola in the capital since Friday, a worrisome increase in infections just over a month after an outbreak was declared in a remote part of the East African country.

Nine more people in the Kampala metropolitan area tested positive for Ebola on Sunday, in addition to two others on Friday, Health Minister Jane Ruth Aceng said Monday.

A top World Health Organization official in Africa said last week that Uganda’s Ebola outbreak was “rapidly evolving,” describing a challenging situation for health workers.

Ugandan health authorities have confirmed 75 cases of Ebola since Sept. 20, including 28 deaths. There are 19 active cases.

The official numbers don’t include those who probably died of Ebola before the outbreak was confirmed in a farming community about 150 kilometers (93 miles) west of Kampala.

Fears that Ebola could spread far from the outbreak’s epicenter compelled authorities to impose an ongoing lockdown, including nighttime curfews, on two of the five districts reporting Ebola cases. The measures were put in place after a man infected with Ebola sought treatment in Kampala and died in a hospital there.

The nine new cases reported Monday follow a similar pattern as they all are contacts of an Ebola-infected patient who traveled from an Ebola hotspot and sought treatment at Kampala’s top public hospital, known as Mulago.

There is no proven vaccine for the Sudan strain of Ebola that’s circulating in Uganda.

Ugandan officials by Thursday had documented more than 1,800 Ebola contacts, 747 of whom had completed 21 days of monitoring for possible signs of the disease that manifests as a viral hemorrhagic fever, according to the Africa Centers for Disease Control and Prevention.

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

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.

Scientists don’t know the natural reservoir of Ebola, but they suspect the first person infected in an outbreak acquired the virus through contact with an infected animal or eating its raw meat. Ugandan officials are still investigating the source of the current outbreak.

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.