As global concerns over the coronavirus outbreak grow, the U.S. is taking steps to prepare for the possibility of a pandemic, the Centers for Disease Control and Prevention said Friday.

Health authorities are closely monitoring the spread of the virus — not only in China, where the outbreak began and where the vast majority of the nearly 77,000 cases have been diagnosed — but also in the growing number of cases in other countries. By definition, a pandemic is an epidemic on more than one continent.

In South Korea, officials said cities were facing an "unprecedented crisis" as the number of cases doubled in a single day, rising to more than 200. Many of the infections diagnosed in Korea are linked to a single church service, said officials who urged residents to stay indoors as much as possible.

On Friday, Lebanon diagnosed its first case. And the Iranian Health Ministry has confirmed 18 cases of the illness, including four deaths.

In the U.S., the number of confirmed cases rose to at least 34, as a number of evacuated individuals from the Diamond Princess cruise ship have tested positive.

Several other Americans who were on board the ship have been diagnosed and hospitalized in Japan, according to the CDC. As of Friday afternoon, there were no plans to evacuate those patients. The U.S. Department of State said it will not rescue other Americans who choose to go on cruises in Asia.

American evacuees from the coronavirus outbreak in China board a bus after arriving by plane to Eppley Airfield in Omaha, Neb., on Feb. 7, 2020. Nati Harnik / AP

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"Such repatriation flights do not reflect our standard practice, and should not be relied upon as an option for U.S. citizens," Ian Brownlee, an executive within the Department of State, said during a media briefing Friday.

On its website, the department has warned Americans to reconsider any planned cruises to or within Asia, saying such trips "may be impacted by travel restrictions affecting their itineraries or ability to disembark, or may be subject to quarantine procedures implemented by the local authorities."

The number of U.S. coronavirus cases is almost certain to grow, as thousands of people who've traveled recently to China are quarantining themselves voluntarily in their homes for 14 days and monitoring for symptoms.

The system appears to be working. A traveler in Sacramento County, California, who was under a self-quarantine was confirmed Friday to have the coronavirus. The patient, who has not yet been included in the CDC's tally of 34, had mild symptoms, according to Sacramento County Public Health.

Full coverage of the coronavirus outbreak

Still, the CDC is preparing in the event the outbreak gets worse.

"We are reviewing all of our pandemic preparedness materials and adapting them to COVID-19," Dr. Nancy Messonnier, director of CDC's National Center for Immunization and Respiratory Diseases, said Friday during a media briefing, using the name of the disease caused by the coronavirus. "The materials will serve as a blueprint for the community interventions that we will use here in the U.S."

That means reinforcing infection control procedures, making sure health care workers have enough supplies, and planning for a possible influx of patients.

There may be a time when more aggressive measures, such as temporarily closing schools or businesses in the U.S., are necessary, she added.

Can the coronavirus be contained?

How countries are able to contain the virus and prevent it from sustained person-to-person transmission will be important in determining the "ultimate end game" of the virus, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told NBC News.

"If infection control gets out of control in many of these countries throughout the world, then it's going to be very difficult to prevent cases from then going, by travel, to all parts of the world," Fauci said. "That's how a pandemic starts."

As part of a team of international scientists led by the World Health Organization, the National Institutes of Health and the CDC have staff members in China, observing containment and infection control efforts. They're scheduled to travel into the epicenter of the outbreak, Wuhan, on Saturday.

"We still believe we can contain the virus," WHO Director-General Dr. Tedros Adhanom Ghebreyesus said Friday during a media briefing. "But the window of opportunity is narrowing."

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