A third American aid worker, Dr. Rick Sacra, to be treated in the U.S. for the deadly Ebola virus arrived Friday at the Nebraska Medical Center's biomedical isolation unit — the largest in the country.

Here are some questions and answers about the Omaha unit:

WHY IS THE COUNTRY'S LARGEST BIOCONTAINMENT UNIT IN OMAHA?

The Nebraska Biocontainment Patient Care Unit got its start in the years after Sept. 11 as Nebraska prepared to combat bioterrorism. By 2004, Nebraska ranked among the top six states for bioterrorism preparedness, according to a report by the nonprofit Trust for America's Health.

A year later, Nebraska's health agency pooled its allotment of federal bioterrorism dollars with contributions from the hospital and the University of Nebraska's medical school and opened the $1 million isolation unit.

The 10-bed, five-room unit is the largest quarantine and treatment facility in the country and designed to handle highly contagious and deadly infections — including severe acute respiratory syndrome (SARS), smallpox and plague. Other biocontainment units are in Montana, Maryland and at Emory University Hospital in Atlanta, where two infected Americans were treated earlier this summer.

HOW MANY PEOPLE HAS THE UNIT TREATED?

The unit has so far briefly housed only one person, a traveler five years ago from Africa whose symptoms concerned emergency-room workers in a Nebraska town, according to unit officials. The patient was diagnosed with malaria, which doesn't require quarantine.

WHY WAS DR. SACRA SENT TO THE OMAHA UNIT, AS OPPOSED TO EMORY'S IN ATLANTA?

The U.S. Department of Health asked the Omaha medical center to treat Sacra in order to prepare other biocontainment units besides Emory's to take more Ebola patients, if needed.

WHAT KIND OF EQUIPMENT DOES OMAHA HAVE?

The biocontainment unit is separate from all other units in the hospital, and medical staff can only enter by security access. The unit has its own air-handling system to help ensure that no infectious particles escape to the largest hospital campus.

Ultraviolet light, a dunk tank for lab specimens and a sterilizer for items to be taken out of the unit are among the safety features designed to protect people on the outside.

The unit also has two videophones that families and friends can use to talk to ill loved ones, as well as for easier and safer medical consultations.

A special transportation gurney enclosed by a bubble-type seal is used to transfer contaminated patients to the unit on the seventh floor.

WHO WORKS THERE?

Dr. Philip Smith is the medical director of the Nebraska Biocontainment Patient Care Unit and a professor of infectious diseases at the University of Nebraska Medical Center. Smith has served as guest faculty for the Hospital Infections Course at the Centers for Disease Control and Prevention in Atlanta; is president of the Nebraska Infection Control Network board; and is co-director of the Nebraska Center for Bioterrorism Education.

Smith will work closely with Dr. Angela Hewlett, associate medical director of the isolation unit and assistant professor of infectious diseases at the center.

Dr. Mark Rupp, as chief of UNMC's Division of Infectious Disease, oversees the treatment at the unit.

A staff of 35 doctors, nurses and other medical professionals will provide Sacra with care.