Every human being is unique. But some are more unique than others, especially when it comes to spreading contagious diseases like SARS or Ebola. They are what epidemiologists term ¿superspreaders.¿ In a report published today in the journal Nature, researchers describe how they combined statistical studies with a mathematical model to determine how superspreaders affect the emergence of new diseases.

It¿s relatively easy to predict how sexually transmitted diseases, such as AIDS will spread, because it¿s possible to estimate who is at risk, and to ask victims how many partners they¿ve had. The same is true of vector-borne ailments such as Lyme disease, which is acquired through the bite of an infected tick. You know where to find ticks, and you usually know when you¿ve been bitten by one. Much more difficult is the task of forecasting the course that diseases transmitted through casual contact, such as smallpox or influenza, will take through a population.

James Lloyd-Smith, a biophysicist at the University of California at Berkeley and colleagues studied the statistics of eight diseases ranging from measles to monkeypox, and were surprised to find that there was no meaningful ¿average¿ number of people who could be infected by a contagious individual. ¿A lot of people don¿t infect anyone,¿ Lloyd-Smith says. Rather, a tiny number of superspreaders are responsible for an epidemic. The researchers developed a mathematical model to predict disease dynamics arising from superspreaders, which showed that depending on circumstances, a viral outbreak can either fizzle out or explode.

"There is a great need for rapid action once a disease is identified, to identify the people you should target for control," Lloyd-Smith says. "SARS was able to spread in China until the Chinese authorities took broad measures to quarantine the sick." There are two ways for a population to protect itself: either everyone can act to reduce their chance of transmission--for example, by staying at home, which can throttle a country¿s economy--or authorities can identify those most likely to be superspreaders, and focus their vaccination and isolation efforts on them.