When we say that a handful of East Village kids started the Hush Puppies epidemic, or that the scattering of the residents of a few housing projects was sufficient to start Baltimore's syphilis epidemic, what we are really saying is that in a given process or system some people matter more than others. This is not, on the face of it, a particularly radical notion. Economists often talk about the 80/20 Principle, which is the idea that in any situation roughly 80 percent of the "work" will be done by 20 percent of the participants. In most societies, 20 percent of criminals commit 80 percent of crimes. Twenty percent of motorists cause 80 percent of all accidents. Twenty percent of beer drinkers drink 80 percent of all beer. When it comes to epidemics, though, this disproportionality becomes even more extreme: a tiny percentage of people do the majority of the work.

Potterat, for example, once did an analysis of a gonorrhea epidemic in Colorado Springs, Colorado, looking at everyone who came to a public health clinic for treatment of the disease over the space of six months. He found that about half of all the cases came, essentially, from four neighborhoods representing about 6 percent of the geographic area of the city. Half of those in that 6 percent, in turn, were socializing in the same six bars. Potterat then interviewed 768 people in that tiny subgroup and found that 600 of them either didn't give gonorrhea to anyone else or gave it to only one other person. These people he called nontransmitters. The ones causing the epidemic to grow — the ones who were infecting two and three and four and five others with their disease — were the remaining 168. In other words, in all of the city of Colorado Springs — a town of well in excess of 100,000 people — the epidemic of gonorrhea tipped because of the activities of 168 people living in four small neighborhoods and basically frequenting the same six bars.

Who were those 168 people? They aren't like you or me. They are people who go out every night, people who have vastly more sexual partners than the norm, people whose lives and behavior are well outside of the ordinary. In the mid-1990s, for example, in the pool halls and roller-skating rinks of Kast St. Louis, Missouri, there was a man named Darnell "Boss Man" McGee. He was big — over six feet — and charming, a talented skater, who wowed young girls with his exploits on the rink. His specialty was thirteen- and fourteen-year-olds. He bought them jewelry, took them for rides in his Cadillac, got them high on crack, and had sex with them. Between 1995 and 1997, when he was shot dead by an unknown assailant, he slept with at least 100 women and — it turned out later — infected at least 30 of them with HIV.

In the same two-year period, fifteen hundred miles away, near Buffalo, New York, another man — a kind of Boss Man clone — worked the distressed downtown streets of Jamestown. His name was Nushawn Williams, although he also went by the names "Face," "Sly," and "Shyteek." Williams juggled dozens of girls, maintaining three or four different apartments around the city, and all the while supporting himself by smuggling drugs up from the Bronx. (As one epidemiologist familiar with the case told me flatly, "The man was a genius. If I could get away with what Williams did, I'd never have to work a day again in my life.") Williams, like Boss Man, was a charmer. He would buy his girlfriends roses, let them braid his long hair, and host all-night marijuana and malt liquor-fueled orgies at his apartments. "I slept with him three or four times in one night," one of his partners remembered. "Me and him, we used to party together all the time… After Face had sex, his friends would do it too. One would walk out, the other would walk in." Williams is now in jail. He is known to have infected at least sixteen of his former girlfriends with the AIDS virus. And most famously, in the book And the Band Played On, Randy Shilts discusses at length the so-called Patient Zero of AIDS, the French-Canadian flight attendant Gaetan Dugas, who claimed to have 2,500 sexual partners all over North America, and who was linked to at least 40 of the earliest cases of AIDS in California and New York. These are the kinds of people who make epidemics of disease tip.

Social epidemics work in exactly the same way. They are also driven by the efforts of a handful of exceptional people. In this case, it's not sexual appetites that set them apart. It's things like how sociable they are, or how energetic or knowledgeable or influential among their peers. In the case of Hush Puppies, the great mystery is how those shoes went from something worn by a few fashion forward downtown Manhattan hipsters to being sold in malls across the country. What was the connection between the East Village and Middle America? The Law of the Few says the answer is that one of these exceptional people found out about the trend, and through social connections and energy and enthusiasm and personality spread the word about Hush Puppies just as people like Gaetan Dugas and Nushawn Williams were able to spread HIV.

In Baltimore, when the city's public clinics suffered cutbacks, the nature of the syphilis affecting the city's poor neighborhoods changed. It used to be an acute infection, something that most people could get treated fairly quickly before they had a chance to infect many others. But with the cutbacks, syphilis increasingly became a chronic disease, and the disease's carriers had three or four or five times longer to pass on their infection. Epidemics tip because of the extraordinary efforts of a few select carriers. But they also sometimes tip when something happens to transform the epidemic agent itself.

This is a well-known principle in virology. The strains of flu that circulate at the beginning of each winter's flu epidemic are quite different from the strains of flu that circulate at the end. The most famous flu epidemic of all — the pandemic of 1918 — was first spotted in the spring of that year and was, relatively speaking, quite tame. But over the summer the virus underwent some strange transformation and over the next six months ended up killing between 20 and 40 million people worldwide. Nothing had changed in the way in which the virus was being spread. But the virus had suddenly become much more deadly.

The Dutch AIDS researcher Jaap Goudsmit argues that this same kind of dramatic transformation happened with HIV. Goudsmit's work focuses on what is known as Pneumocystis carinii pneumonia, or PCP. All of us carry the bacterium in our bodies, probably since birth or immediately thereafter. In most of us it is harmless. Our immune systems keep it in check easily. But if something, such as HIV, wipes out our immune system, it becomes so uncontrollable that it can cause a deadly form of pneumonia. PCP is so common among AIDS patients, in fact, that it has come to be seen as an almost certain indication of the presence of the virus. What Goudsmit did was go back in the medical literature and look for cases of PCP, and what he found is quite chilling. Just alter World War II, beginning in the Baltic port city of Danzig and spreading through central Europe, there was an epidemic of PCP that claimed the lives of thousands of small children.

Goudsmit has analyzed one of the towns hit hardest by the PCP epidemic, the mining town of Heerlen in the Dutch province of Limburg. Heerlen had a training hospital for midwives called the Kweekschool voor Vroedvrouwen, a single unit of which — the so-called Swedish barrack — was used in the 1950s as a special ward for underweight or premature infants. Between June 1955 and July 1958, 81 infants in the Swedish barrack came down with PCP and 24 died. Goudsmit thinks that this was an early HIV epidemic, and that somehow the virus got into the hospital, and was spread from child to child by the then, apparently common, practice of using the same needles over and over again for blood transfusions or injections of antibiotics. He writes: