when can the government quarantine its citizens?

Slate | When someone has been exposed to diphtheria, infectious tuberculosis, yellow fever, viral hemorrhagic fevers, SARS, and a new strain of influenza with pandemic potential. That's the federal government's list, which changes only by executive order; states may have their own. Patients who have actually been infected, like the Atlanta man [Andrew Speaker], are technically under "isolation," not quarantine.

The federal government took responsibility for isolating Speaker because his plane travel made his case an interstate and international health threat. Normally, the states have primary responsibility for quarantine and isolation. Some state laws specify which illnesses require quarantine, while others focus on public-health threats in general. In Louisiana, health officials need permission from a judge before confining a patient. Any people in Minnesota who lose their jobs because of a quarantine or isolation can sue their employers.

States have been broadening their public-health powers in recent years because of bioterrorism and pandemic fears. Sometimes a state will amend its laws to deal with a specific crisis. For instance, Alaska had to pass an amendment during the SARS scare to be able to quarantine and investigate a docked ship.

In certain states, disobeying a quarantine or isolation order can put you behind bars. A 27-year-old man in Arizona diagnosed with the same deadly strain of [tuberculosis] has been quarantined in a prison hospital ward for the past 10 months; state officials said they put Robert Daniels there because he failed to take his medicine and endangered others by going out and entertaining friends without wearing a mask.

But in general, the government rarely has to exercise these powers, because most of the time, citizens cooperate. In fact, until this recent situation, the CDC hadn't issued such an order since 1963, when it quarantined a woman for smallpox exposure.* Even during the SARS epidemic in 2003, officials relied mostly on voluntary isolation and quarantine. And the last large-scale quarantine in the United States took place during the Spanish flu epidemic of 1918-19.

Bonus Explainer: What happens in isolation? A patient often lives in a room with negative air pressure to prevent disease from spreading; an ultraviolet light may kill potential pathogens as they're sucked through a HEPA filter. Visitors must wear masks or, in some cases, sterile full-body suits.