The Centers for Disease Control and Prevention said on Friday that a U.S. citizen had tested positive for Ebola in the Democratic Republic of Congo, where an outbreak of the disease has killed hundreds of people this year.

The American, a worker with a humanitarian organization in Congo, contracted Ebola from the Bundibugyo virus, the C.D.C. said. Little is known about the Bundibugyo pathogen, one of three viral species known to cause Ebola, and there are no targeted vaccines or treatment for it.

The C.D.C. said that it was working with the person’s employer, U.S. agencies, public health authorities and Congolese partners to prevent further transmission and identify close contacts.

The last U.S. citizen who tested positive for the virus during the ongoing outbreak was Dr. Peter Stafford, a medical missionary who contracted it in May while treating patients in Congo. He was transported to Charité University Hospital in Berlin and returned to his home in the United States after recovering, according to Serge, his employer.

The Congolese health authorities said this week that more than 600 people had died from Ebola out of nearly 1,800 cases. The virus has also spread to Uganda, which reported 20 confirmed cases including two deaths, the World Health Organization said. The Trump administration’s aid cuts shuttered crucial disease surveillance systems and medical supply chains that could have detected and contained the outbreak sooner.

Secretary of State Marco Rubio said in May that no Ebola patients would be allowed to enter the United States after the W.H.O. announced the outbreak in Congo. Mr. Rubio later appeared to soften that stance.

U.S. government officials later told reporters that instead of bringing home Americans exposed to Ebola, the Trump administration planned to send them to Kenya, where the construction of a quarantine facility was planned. That prompted a fierce outcry in Kenya, where hundreds of people protested it. A Kenyan court also ordered the suspension of those plans.

Trump administration officials have not articulated a clear plan since then for caring for Americans abroad who have been exposed to the virus.