When can a government quarantine its citizens? According to the article:

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, are technically under "isolation," not quarantine.