I remember once as a child seeing our family's puppy encounter snow (or the first time. He was shocked and delighted and overwhelmed, wagging his tail nervously, sniffing about in this strange, fluffy substance, whimpering with the mystery of it all. It wasn't much colder on the morning of his first snowfall than it had been the evening before. It might have been 34 degrees the previous evening, and now it was 31 degrees. Almost nothing had changed, in other words, yet — and this was the amazing thing — everything had changed. Rain had become something entirely different. Snow! We are all, at heart, gradualists, our expectations set by the steady passage of time. But the world of the Tipping Point is a place where the unexpected becomes expected, where radical change is more than possibility. It is — contrary to all our expectations — a certainty.

In pursuit of this radical idea, I'm going to take you to Baltimore, to learn from the epidemic of syphilis in that city. I'm going to introduce three fascinating kinds of people I call Mavens, Connectors, and Salesmen, who play a critical role in the word-of-mouth epidemics that, dictate our tastes and trends and fashions. I'll take you to the set of the children's shows Sesame Street and Blue's Clues and into the fascinating world of the man who helped to create the Columbia Record Club to look at how messages can be structured to have the maximum possible impact on all their audience. I'll take you to a high-tech company in Delaware to talk about the Tipping Points that govern group life and to the subways of New York City to understand how the crime epidemic was brought to an end there. The point of all of this is to answer two simple questions that lie at the heart of what we would ail like to accomplish as educators, parents, marketers, business people, and policymakers. Why is it that some ideas or behaviors or products start epidemics and others don't? And what can we do to deliberately start and control positive epidemics of our own?

In the mid-1990s, the city of Baltimore was attacked by an epidemic of syphilis. In the space of a year, from 1995 to 1996, the number of children born with the disease increased by 500 percent. If you look at Baltimore's syphilis rates on a graph, the line runs straight for years and then, when it hits 1995, rises almost at a right angle.

What caused Baltimore's syphilis problem to tip? According to the Centers for Disease Control, the problem was crack cocaine. Crack is known to cause a dramatic increase in the kind of risky sexual behavior that leads to the spread of things like HIV and syphilis. It brings far more people into poor areas to buy drugs, which then increases the likelihood that they will take an infection home with them to their own neighborhood. It changes the patterns of social connections between neighborhoods. Crack, the CDC said, was the little push that the syphilis problem needed to turn into a raging epidemic.

John Zenilman of Johns Hopkins University in Baltimore, an expert on sexually transmitted diseases, has another explanation: the breakdown of medical services in the city's poorest neighborhoods. "In 1990-91, we had thirty-six thousand patient visits at the city's sexually transmitted disease clinics," Zenilman says. "Then the city decided to gradually come back because of budgetary problems. The number of clinicians [medical personnel] went from seventeen to ten. The number of physicians went from three to essentially nobody. Patient visits dropped to twenty-one thousand. There also was a similar drop in the amount of held outreach staff. There was a lot of politics — things that used to happen, like computer upgrades, didn't happen. It was a worst-case scenario of city bureaucracy not functioning. They would run out of drugs."

When there were 36,000 patient visits a year in the STD clinics of Baltimore's inner city, in other words, the disease was kept in equilibrium. At some point between 36,000 and 21,000 patient visits a year, according to Zenilman, the disease erupted. It began spilling out of the inner city, up the streets and highways that connect those neighborhoods to the rest of the city. Suddenly, people who might have been infectious for a week before getting treated were now going around infecting others for two or three or four weeks before they got cured. The breakdown in treatment made syphilis a much bigger issue than it had been before.

There is a third theory, which belongs to John Potterat, one of the country's leading epidemiologists. His culprits are the physical changes in those years affecting East and West Baltimore, the heavily depressed neighborhoods on either side of Baltimore's downtown, where the syphilis problem was centered. In the mid-1990s, he points out, the city of Baltimore embarked on a highly publicized policy of dynamiting the old 1960s-style public housing high-rises in East and West Baltimore. Two of the most publicized demolitions — Lexington Terrace in West Baltimore and Lafayette Courts in East Baltimore — were huge projects, housing hundreds of families that served as centers for crime and infectious disease. At the same time, people began to move out of the old row houses in East and West Baltimore, as those began to deteriorate as well.

"It was absolutely striking," Potterat says, of the first time he toured East and West Baltimore. "Fifty percent of the row houses were boarded up, and there was also a process where they destroyed the projects. What happened was a kind of hollowing out. This fueled the diaspora. For years syphilis had been confined to a specific region of Baltimore, within highly confined sociosexual networks. The housing dislocation process served to move these people to other parts of Baltimore, and they took their syphilis and other behaviors with them."

What is interesting about these three explanations is that none of them is at all dramatic. The CDC thought that crack was the problem. But it wasn't as if crack came to Baltimore for the first time in 1995. It had been there for years. What they were saying is that there was a subtle increase in the severity of the crack problem in the mid-1990s, and that change was enough to set off the syphilis epidemic. Zenilman, likewise, wasn't saying that the STD clinics in Baltimore were shut down. They were simply scaled back, the number of clinicians cut from seventeen to ten. Nor was Potterat saying that all Baltimore was hollowed out. All it took, he said, was the demolition of a handful of housing projects and the abandonment of homes in key downtown neighborhoods to send syphilis over the top. It takes only the smallest of changes to shatter an epidemic's equilibrium.

The second, and perhaps more interesting, fact about these explanations is that all of them are describing a very different way of tipping an epidemic. The CDC is talking about the overall context for the disease — how the introduction and growth of an addictive drug can so change the environment of a city that it can cause a disease to tip. Zenilman is talking about the disease itself. When the clinics were cut back, syphilis was given a second life. It had been an acute infection. It was now a chronic infection. It had become a lingering problem that staved around for weeks. Potterat, for his part, was focused on the people who were carrying syphilis. Syphilis, he was saying, was a disease carried by a certain kind of person in Baltimore — a very poor, probably drug-using, sexually active individual. If that kind of person was suddenly transported from his or her old neighborhood to a new one — to a new part of town, where syphilis had never been a problem before — the disease would have an opportunity to tip.

There is more than one way to tip an epidemic, in other words. Epidemics are a function of the people who transmit infectious agents, the infectious agent itself, and the environment in which the infectious agent is operating. And when an epidemic tips, when it is jolted out of equilibrium, it tips because something has happened, some change has occurred in one (or two or three) of those areas. These three agents of change I call the Law of the Few, the Stickiness Factor, and the Power of Context.