A doctor with the World Health Organization in Sierra Leone has tested positive for the Ebola virus.

The physician is in stable condition in Freetown and will be relocated shortly, the UN health agency said in a statement Monday.

To maintain patient confidentiality, WHO spokeswoman Nyka Alexander in Freetown said the agency won't provide details on nationality or where the person is going for treatment.

The doctor was working at the Ebola treatment centre in the Kenema Government Hospital, which is run by Sierra Leone's health ministry.

The physician had symptoms on Saturday that were confirmed yesterday as Ebola, and the evacuation is scheduled for today, Alexander said.

To ensure the safety of those inside the hospital, WHO recommended to the health ministry that new admissions be limited while an investigation is launched.

"Every time a health-care worker is infected, it's very worrisome," Alexander said.

"What we're seeing from some of the investigations is it's not always an infection within the facility. You can't be sure that the infection didn't happen within the community, for example, or outside of the ward."

Given there are so many questions, investigations are important to try to figure out what happened and to make nobody else is at risk, Alexander added.

Meanwhile, a fourth American who contracted Ebola in West Africa is expected to arrive in the U.S. for care Tuesday and will be treated at an Atlanta hospital where two other aid workers successfully recovered from the disease, Emory University Hospital said Monday. The hospital cited confidentiality in not releasing more information about the person.

The State Department said the doctor who contracted Ebola was from the U.S. It is unclear if that is the same person as the American who will be treated at Emory University Hospital.

Previously, a Senegalese epidemiologist working for WHO was infected in Sierra Leone. The epidemiologist is under treatment in Germany for the infection.

Meanwhile in Liberia, Ebola cases are rising "exponentially," WHO said. 

The UN health agency gave an update on Monday after its team of emergency experts worked together with Liberian President Ellen Johnson Sirleaf and members of her government to assess the country's Ebola situation.

"Transmission of the Ebola virus in Liberia is already intense and the number of new cases is increasing exponentially."

"Liberia, together with the other hard-hit countries, namely Guinea and Sierra Leone, is experiencing a phenomenon never before seen in any previous Ebola outbreak. As soon as a new Ebola treatment facility is opened, it immediately fills to overflowing with patients, pointing to a large but previously invisible caseload," WHO said in a statement.

Liberia has the most cases and deaths to date. As of Monday, there were nearly 2,000 cases and more than 1,000 deaths.

A total of 152 health-care workers have been infected and 79 have died.

Both motorbike taxis and regular taxis are "a hot source of potential virus transmission" in Liberia, WHO said.

Elsewhere, Britain said on Monday it would send military and humanitarian experts to Sierra Leone to set up a treatment centre for patients infected with the Ebola virus.

The U.S. Pentagon also announced it will send a 25-bed field hospital to Liberia to help provide medical care
for health workers.

Ebola vaccine studies

In terms of potential treatments, a new study on monkeys suggested that one shot of an Ebola vaccine protected animals exposed to high levels of the virus, but they needed a booster shot.

A single dose protected all four vaccinated monkeys when they were exposed to high levels of Ebola virus five weeks
later, researchers reported Sunday in the journal Nature Medicine. 

When monkeys were exposed to Ebola 10 months after vaccination, only half were protected. If the animals were given a booster with a different formula, they continued to have full protection.

Britain-based GlaxoSmithKline plans to manufacture up to 10,000 doses of the vaccine, developed by the U.S. National Institutes of Health. It is based on a chimpanzee adenovirus.

Canadian researchers created a similar Ebola vaccine that works in monkeys. Manufacturer NewLink Genetics of Ames, Iowa, said a small safety study in healthy volunteers is set to begin in a few weeks.

Gary Kobinger, head of special pathogens at the National Microbiology Laboratory in Winnipeg, helped to develop the other vaccine, which is based on a livestock virus called vesicular stomatitis virus or VSV.

Kobinger told reporters at the American Society for Microbiology meeting in Washington on Monday that safety studies are important to help understand the benefits and risks of potential vaccines.

Kobinger was hopeful that experimental vaccines and drugs, such as the ZMapp cocktail, could offer more options to help bring future Ebola outbreaks under control faster.

A new team from the Winnipeg lab has returned to Sierra Leone to help diagnose or rule out cases of Ebola. A previous team was pulled after questions remained about their safety at a hotel where people were infected.

"The lab will very much be welcome when it's back again," Alexander said.

Without the Canadian scientists, doctors have had to send lab samples on a long, bumpy road to Kenema.

Kobinger said the hotel was completely cleaned and is now managed by Doctors Without Borders, and he's confident the risk level for his staff there is very low.

With files from CBC's Karen Pauls, Reuters and The Associated Press