Quarantine, from the Italian "quarantina," which means "space of 40 days," dates from 15th-century regulations devised in certain Italian cities to control the spread of plague by sequestering those thought to have been exposed to the disease. Along with isolation -- secluding those who are clearly sick -- it can be an effective tool for controlling outbreaks of certain types of disease. In 1910 and 1920, before antibiotics, plague experts in Manchuria controlled several deadly outbreaks of pneumonic plague using quarantine and isolation alone. But pneumonic plague, now rare, spreads in a very different way than flu does. Pneumonic plague germs are coughed out in large droplets that quickly fall to the ground. If you are more than six feet away from a plague patient, you're unlikely to catch the disease. Also, plague patients are typically very ill before they can transmit the germ to others. "There is no disease more susceptible to quarantine than plague," wrote the physician Wu Lien-teh, who helped break the Manchurian epidemics.
Influenza is entirely different. The virus spreads explosively. Coughing, sneezing, or even speaking launches flu particles in an aerosol cloud of tiny droplets, which can drift in the air for some distance. Physician and flu researcher Edwin Kilbourne, who worked with flu patients during the pandemic of 1957-58, points out that people with flu may shed the virus even before they know they're sick -- not much, but enough to transmit the disease. Worse, some 10 to 20 percent of flu patients have subclinical infections; they never look sick at all. Yet they can still spread infection. Faced with a flu pandemic, you'd hardly know where the disease was coming from.
How can you quarantine a disease like that? According to Kilbourne, you can't. "I think it is totally unreasonable on the basis of every pandemic we've had," says Kilbourne. "Every earlier pandemic seeded in multiple foci at the same time. Quarantine simply will not work."
Indeed, a strictly enforced quarantine could do more harm than good. Herding large numbers of possibly infected people together makes it likely that any influenza strain passed among them would actually increase in virulence. Usually, in order to spread, human flu germs need hosts mobile enough to walk around and sneeze on other people. Those flu strains so deadly that they kill or disable their hosts won't get the chance to spread and will die off. This keeps human flu virulence within bounds.
The signal exception is the 1918 flu, which acquired its extreme lethality, according to University of Louisville evolutionary biologist Paul W. Ewald, in the crowded and terrible conditions on the Western Front during World War I. Troops by the train and truckload were constantly being moved in and out of this petri dish, meaning a severely flu-stricken soldier didn't have to move much to infect others.
Suppose that a government official today decided to round up exposed people and move them to a space like the Superdome in New Orleans. It's unlikely that even a crowded Superdome could replicate the conditions on the Western Front. But, depending on how densely packed people were, you could expect the flu strain trapped among them to increase in virulence. You'd be breeding a deadlier flu.
If you let people walk around freely, only those strains mild enough to allow people to stay on their feet would spread easily.