195 Americans are quarantined because of coronavirus. What does that mean?

The 195 Americans evacuated from Wuhan, China, on a plane chartered by the U.S. government, as part of an effort to prevent the spread of coronavirus, have been placed under a 14-day quarantine.

After that decision by the Centers for Disease Control and Prevention, the White House on Friday declared the 2019 novel coronavirus a public health emergency. Additionally, any U.S. citizens returning from mainland China are being required to undergo proactive health screenings at ports of entry and spend up to 14 days in self-quarantine.

These announcement came a day after the World Health Organization declared the coronavirus a global health emergency. To date, seven cases in the U.S. have been confirmed, but almost 12,000 have been reported in China, where at least 259 have died from the disease.

"While we recognize this is an unprecedented action, we are facing an unprecedented public health threat, and this is one of the tools in our toolbox to mitigate the potential impact of this virus in the United States," Dr. Nancy Messonier, director of the CDC's National Center for Immunization and Respiratory Diseases, said at a press briefing.

It's the first time in more than 50 years the CDC has issued such a quarantine.

"While there have been several isolation orders over the last many years, the last time the quarantine was used for a suspect case was in the 1960s for a smallpox evaluation," said Dr. Marty Cetron, director of CDC's Division of Global Migration and Quarantine.

Isolation and quarantine are two different tools used to restrict the spread of disease. Using isolation is much more common.

Both are legal measures available to federal and state authorities to limit exposure or prevent a potentially contagious disease from spreading. In most states, breaking a quarantine order is a criminal misdemeanor.

"'Isolation' is the legal term that refers to separating someone who is already sick, or reasonably believed to be, from healthy people, while 'quarantine' is used for isolating people that are not yet sick and may never get sick" but may have been exposed to something contagious, said Polly Price, a law and public health professor at Emory University.

The CDC is also authorized to detain, medically examine and release people arriving in the United States or travelling between states if they're potentially infectious. The CDC routinely monitors people crossing U.S. borders for signs and symptoms of communicable diseases and may order a federal isolation or quarantine if an individual is deemed potentially infectious.

With the coronavirus, health officials currently believe symptoms will manifest within two weeks, so the quarantines effectively cover the incubation period for the disease, researchers believe. So far, specimen samples from passengers entering the U.S. have been negative. But that's not a guarantee.

"The testing we have available now cannot identify people in the incubation phase. This is a point-in-time, single test, and shouldn't be relied upon to make a prediction as to whether or not the person will become ill," said Price, echoing sentiments from the CDC. "Until we have a better test to more rapidly determine whether someone is carrying the virus, quarantine is the best tool we have. We have to trust the government is using the best science available to institute the minimum restrictions."

But, Cetron, added, "Clearly, there are cons."

If improperly enacted, quarantines can "induce fear and stigma of individuals. If people aren't treated with dignity and respect and have all the supportive care, that's not the way to implement this tool."

Not all individuals have been cooperative, CDC officials said. One person reportedly tried to leave, and officials issued a state-level California quarantine to enhance the level of protection. Authorities declined to comment further.

"We'd rather be remembered for overreacting than underreacting," Messonier said. "We are preparing as if this were the next pandemic, but we are hopeful still that this is not and will not be the case."

Eden David, who studies neuroscience at Columbia University and is matriculating to medical school later this year, is a member of the ABC News Medical Unit.