ATLANTA (AP) — A second American medical missionary stricken with the often deadly Ebola virus is expected to be flown Tuesday to the U.S. for treatment, following a colleague who was admitted over the weekend to Emory University Hospital’s infectious disease unit.

Top American public health officials continue to emphasize that treating Nancy Writebol and Dr. Kent Brantly in the U.S. poses no risks to the public as West Africa grapples with its worst recorded Ebola outbreak in history.

“The plain truth is that we can stop Ebola,” said Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, speaking Sunday on ABC’s “This Week. “We know how to control it: hospital infection control and stopping it at the source in Africa.”

Brantley and Writebol served on the same medical mission team that was treating Ebola patients in Liberia. Also spreading in Guinea and Sierra Leone, the outbreak has infected more than 1,300 people in West Africa, killing at least 729 of them.

Liberian officials said a medical evacuation plane would transport Writebol to the United States early Tuesday. Information Minister Lewis Brown told The Associated Press that the flight was expected to leave West Africa between at 1 a.m. and 1.30 a.m. local time Tuesday.

Brantly arrived Saturday under the same protocol, flying from West Africa to Dobbins Air Reserve base outside Atlanta in a small plane equipped to contain infectious diseases. A small police escort followed his ambulance to the hospital, where he emerged dressed head to toe in white protective clothing and walked into the hospital on his own power.

In another television appearance, Frieden said on “Fox News Sunday” that Brantly “appears to be improving.”

An American mission official has said Brantly was treating victims of the outbreak at a hospital compound near Monrovia, Liberia, when he became infected. They said Writebol served as a hygienist whose role included decontaminating those entering or leaving the Ebola treatment area at that hospital.

There is no cure for the Ebola virus, which causes hemorrhagic fever that kills at least 60 percent of the people it infects in Africa. It is spread by close contact with bodily fluids and blood, meaning it is not spread as easily as airborne influenza or the common cold.

That means any modern hospital using standard infection-control measures should be able to handle it. American doctors say the virus could be curtailed in Africa by a better functioning health care system.

Emory officials have not commented on Brantly’s condition. And no immediate details were provided by U.S. health officials for Writebol’s planned treatment.

The hospital’s infectious disease unit is one of about four in the country equipped to test and treat people exposed to dangerous viruses.

Patients are quarantined, sealed off from anyone who is not in protective gear. Lab tests are conducted inside the unit, ensuring that viruses don’t live the quarantined area.

Family members can see and communicate with patients only through barriers.

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Larson reported from Dakar, Senegal.