Epidemics have their own set of rules. Suppose, for example, that one summer a thousand tourists come to Manhattan from Canada carrying an untreatable strain of twenty-four-hour flu. The virus has a two-per-cent infection rate, which is to say that one out of every fifty people who come into close contact with someone carrying it catches the bug himself. Let's say that fifty is also exactly the number of people the average Manhattanite—in the course of riding the subways and mingling with colleagues at work—comes into contact with every day. What we have, then, given the recovery rate, is a disease in equilibrium. Every day, each carrier passes on the virus to a new person. And the next day those thousand newly infected people pass on the virus to another thousand people, so that throughout the rest of the summer and the fall the flu chugs along at a steady but unspectacular clip.

But then comes the Christmas season. The subways and buses get more crowded with tourists and shoppers, and instead of running into an even fifty people a day, the average Manhattanite now has close contact with, say, fifty-five people a day. That may not sound like much of a difference, but for our flu bug it is critical. All of a sudden, one out of every ten people with the virus will pass it on not just to one new person but to two. The thousand carriers run into fifty-five thousand people now, and at a two-per-cent infection rate that translates into eleven hundred new cases the following day. Some of those eleven hundred will also pass on the virus to more than one person, so that by Day Three there are twelve hundred and ten Manhattanites with the flu and by Day Four thirteen hundred and thirty-one, and by the end of the week there are nearly two thousand, and so on up, the figure getting higher every day, until Manhattan has a full-blown flu epidemic on its hands by Christmas Day.

In the language of epidemiologists, fifty is the "tipping point" in this epidemic, the point at which an ordinary and stable phenomenon—a low-level flu outbreak—can turn into a public-health crisis.
—Malcolm Gladwell, "The Tipping Point," The New Yorker, June 3, 1996

The study said, however, that cross-district busing is seldom popular. It noted that whites sometimes oppose it, arguing that school boards are caving in to blacks and civil libertarians. Blacks, on the other hand, dislike busing programs because their children do most of the riding.

The study also stated that the massive busing in Louisville and Nashville did not necessarily mean that it was beneficial educationally.

"What it does mean is that the countywide solution . . . has kept the proportion of black enrollees in the individual schools at a level below the 'tipping point,' thereby minimizing the outflow of white students," the report concluded.
—The Associated Press, November 8, 1977