Plus, when can the government quarantine its citizens?

Strains of swine flu have been confirmed in the United States, Mexico, and Canada, prompting the United States to declare a public health emergency. The particularly virulent strain is believed to have caused the deaths of up to 103 people in Mexico and infected more than 1,000, causing heightened concern of a potential epidemic in countries around the world. In a 2003 "Explainer," Brendan I. Koerner teased out the difference between an outbreak, an epidemic, and a pandemic. In a 2007 Explainer, Michelle Tsai detailed under what conditions the government can quarantine its citizens. The columns are reprinted below.

What's the difference between an outbreak and an epidemic, and where do pandemics fit into the nomenclature of disease?

In terms of the flu, the difference between an outbreak and an epidemic is the percentage of overall deaths caused by the disease. Every week, the CDC gathers morbidity data from hospitals in 122 cities nationwide and figures out what percentage of the decedents died of pneumonia, cancer, and other prolific killers. If the number of flu-caused deaths exceeds 7.7 percent of the total, then the United States officially has an epidemic on its hands. According to the latest survey, only 7.2 percent of last week's deaths were due to influenza, so there's no epidemic yet.

The 7.7 percent figure isn't static from year to year, however. The CDC's official definition of an epidemic is: "The occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time." Since some diseases become more prevalent or lethal over time, while others become less severe, the CDC must adjust its statistical models to alter the definition of what's truly more than expected. During the flu epidemic of 1990, for example, the CDC's threshold was 6.7 percent of total deaths. Today's threshold figure is likely due in part to the emergence of new, less treatable flu strains, as well as the decline in morbidity associated with other diseases.

For diseases that cause a statistically insignificant number of deaths each week, the CDC often uses incidence data in lieu of fatality percentages. The rule of thumb for meningococcal disease, for example, is that an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks constitutes a sizable epidemic. In countries with more centralized health care systems, such as the United Kingdom, the measurement is usually done by surveying the number of complaints made to general practitioners. In the United Kingdom, if 1,000 out of every 100,000 GP consultations are flu-related, then it's an epidemic.

The CDC doesn't have epidemic thresholds for every disease and actually tends to frown on some of the media hairsplitting over the outbreak-epidemic line. The center's online glossary states that outbreak can actually be used interchangeably with epidemic and that the former is often preferable merely because it's a less frightening term.

Few epidemiological words stir up more panic than pandemic, and for good reason. A pandemic is an epidemic that occurs across several countries and affects a sizable portion of the population in each, although there's no formal definition of what constitutes "sizable." According to the CDC, the last influenza pandemic took place in 1968-69, when the Hong Kong flu killed 33,800 Americans between September and March.

When can the government quarantine its citizens?

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.