Government-imposed quarantines were fairly common in ancient times, before medicine stemmed the ferocity with which contagious diseases spread. The very word quarantine is rooted in the Italian words quarantenara and quaranta giorni, or 40 days, the period of time that the city of Venice forced ship passengers and cargo to wait before landing in the 14th and 15th centuries to try to stave off the plague. Since then, quarantines have often generated tensions between protecting public health versus respecting individual rights.

Here are answers to some common questions about how quarantines are imposed and enforced in the United States in the wake of the coronavirus.

Can the government impose a quarantine on anybody?

The legal authority to impose quarantines on individuals is rooted in the “police powers” granted broadly to states, counties and cities to protect public health. That means for most Americans, a state or local quarantine imposed to prevent the spread of the coronavirus will be far more important than any federal order.

When it comes to the federal government, it can impose quarantines under the Public Health Service Act for two main reasons: to prevent the spread of communicable disease into the United States or between states. That is why, for example, the Centers for Disease Control and Prevention ordered Americans flying home after visiting mainland China or Iran to fly into one of 11 major U.S. airports that had the ability to screen passengers.

Quarantines are considered a measure of last resort when no preferable means is available to halt the spread of a deadly communicable disease. Those subject to quarantine should be either infectious or have been exposed to the disease, experts said.

“We do not want to restrict people’s liberty unless it is necessary, unless we cannot achieve the public health end with less draconian measures,” said Wendy E. Parmet, the director of the Center for Health Policy and Law at Northeastern University.

So there is no blanket law?

No, the laws vary by state and even locality. Some 40 states updated their quarantine laws after fears spread over a possible broad anthrax attack in the months after the Sept. 11 attacks, according to Lawrence O. Gostin, a professor at Georgetown University Law School who specializes in public health law. He is the author of a legal template called the State Emergency Health Powers Act, which many states adopted in whole or in part. Some states retain antiquated regulations on their books.

Are quarantines, including self-isolating at home, voluntary or mandatory?

It is a bit of a gray area. It often seems voluntary until the person involved tries to leave, at which point health officials are likely to make it compulsory, experts said. The rules are notoriously hard to confirm because county governments often do not publish their regulations online.

The bottom line, however, is that if a quarantine is not enforced and other people catch the fatal disease as a result, the local government could be held liable, Mr. Gostin said. “Thinking about this as purely voluntary is wishful thinking.”

Is it a crime to evade a quarantine?

Again, laws vary by state, but those who ignore the rule could face fines or jail time. Logic dictates that draconian enforcement would be difficult and often counterproductive. No local law enforcement agency would likely compound its problems by throwing a quarantine scofflaw with a deadly communicable disease in among its jail population.

Local authorities often have some form of enforcement power, but usually try gentle persuasion to persuade people that it is for their good and the good of the community. An infected person blatantly ignoring an order might be forced to go into medical isolation — that is, some form of locked hospital ward.

Experts worry that many Americans might think they have the right to go someplace local like the supermarket without considering the consequences for others. “We have lost this tradition of the common good and social responsibility to each other and that could be a big problem in America,” Mr. Gostin said.

Is there a right to appeal?

States should have some manner of appeal process, and some require a court order from the outset. If there is no medical tribunal or other means for a second opinion, ultimately anyone could challenge a quarantine order in court through a writ of habeas corpus.

Quarantine laws tend to be controversial because they are akin to jail time, using the coercive power of the state to tell people that they have to stay confined, even if in their own homes.

The C.D.C. rewrote its quarantine guidelines in 2017 and they have never been tested in court. The Supreme Court has also never dealt with an infectious disease quarantine case, Mr. Gostin said.

Under C.D.C. rules, the federal government must test those confined within 72 hours and define the length of stay from the outset — two weeks for the coronavirus because that is the incubation period for the disease.

The most famous recent test case was Kaci Hickox, a nurse who was initially quarantined involuntarily at Newark Liberty International Airport in 2014 upon returning from West Africa, where she had worked with Ebola patients.

After a few days, she was allowed to return to her home state of Maine but ordered to remain in isolation. Having tested negative for the virus, Ms. Hickox sued and the judge rejected the quarantine order.

With the help of the A.C.L.U., Ms. Hickox also sued New Jersey, which resulted in a settlement that gave arriving passengers more rights, including the right to appeal the decision and to seek legal advice.