OMAHA, Neb. — An American photojournalist who contracted Ebola while working in West Africa began his journey home for treatment Sunday, while a man who recently arrived in Dallas from Liberia remained in critical condition with the disease.

Ashoka Mukpo, 33, will be the second Ebola patient to be treated at the Nebraska Medical Center’s specialized isolation unit.

Mukpo was working as a freelance cameraman for NBC News in Liberia when he became ill last week. NBC reported Sunday evening that Mukpo had started his journey to the U.S. for treatment and that he would arrive Monday morning. Mukpo’s family said Friday he would be treated in Omaha. Hospital officials said they expected an Ebola patient to arrive Monday, but declined to provide a name.

He is the fifth American to return to the United States for treatment since the start of the latest Ebola outbreak, which the World Health Organization estimates has killed more than 3,400 people.

The hospital’s biocontainment unit was created in 2005 specifically to handle illnesses like this, said Dr. Phil Smith, who oversees the unit.

“We are ready, willing and able to care for this patient,” Smith said. “We consider it our duty to give these American citizens the best possible care we can.”

Mukpo’s father, Dr. Mitchell Levy, told NBC Sunday that his son was “counting the minutes” until he could leave Liberia but that he was not feeling that ill Sunday. Levy said the family was travelling from Rhode Island to Nebraska.

Doctors at the isolation unit — the largest of four nationwide — would evaluate Mukpo when he arrives before determining how to treat him. They said they will apply the lessons learned while treating American aid worker Rick Sacra in September. Sacra was successfully treated in the Nebraska unit and was allowed to return to his home in Massachusetts after three weeks, on Sept. 25.

“Truly, focusing on symptom management is key with these patients,” Dr. Rosanna Morris said Friday.

Sacra received an experimental Tekmira Pharmaceuticals drug called TKM-Ebola, as well as two blood transfusions from another American aid worker who recovered from Ebola at an Atlanta hospital. The transfusions are believed to help a patient fight off the virus because the survivor’s blood carries antibodies for the disease. Sacra also received supportive care, including IV fluids and aggressive electrolyte management.

In Dallas, Thomas Eric Duncan was listed in critical condition Sunday. Duncan has been hospitalized at Texas Health Presbyterian Hospital for one week. Dr. Tom Frieden, director of the federal Centers for Disease Control and Prevention, said he was aware that Duncan’s health had “taken a turn for the worse,” but he declined to describe Duncan’s condition further.

Duncan arrived in Dallas from Liberia on Sept. 20 and fell ill a few days later.

Four members of a family who hosted Duncan in their northeast Dallas apartment are being kept in isolation, though they have not shown symptoms of infection. Ten people definitely had close contact with Duncan and a further 38 may have been around him when he was showing symptoms of the disease, officials said.

The virus that causes Ebola is not airborne and can only be spread through direct contact with the bodily fluids — blood, sweat, vomit, feces, urine, saliva or semen — of an infected person who is showing symptoms.

Earlier Sunday, authorities found a homeless man who may have had contact with Duncan, Dallas city spokeswoman Sana Syed said.

Dallas County Judge Clay Jenkins called the man, whom he didn’t identify, a “low-risk individual,” and said he would be placed in housing that would allow health workers direct access to him daily.

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Associated Press writer Nomaan Merchant in Dallas contributed to this report.