NORFOLK, Va. - Scientists are discovering that a tiny segment of the population, known as "superspreaders," is the driving force behind the transmission of infectious diseases.
Perhaps the most infamous superspreader was Mary Mallon, aka Typhoid Mary. An Irish cook in New York City in the 1900s, she was chronically infected with Salmonella typhi.
Although the infection didn't cause any symptoms for Mallon, she did excrete large numbers of typhoid bacteria in her feces. Her career as a cook made it easy to transmit fecal bacteria to customers through the food she prepared.
She infected about 50 people and killed several (official counts vary) before she was arrested and jailed for refusing to give up her career as a cook.
Originally epidemiologists believed that people like Typhoid Mary were an anomaly and not the main drivers of infectious disease spread.
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But then came severe acute respiratory syndrome, or SARS.
This pandemic started as just another strange pneumonia from southern China, but in 2003 it turned into a global outbreak that infected 8,098 people and killed 774.
Key to the disease's spread, researchers found, was a small but crucial portion of the population of superspreaders, people who transmitted the infection to a much greater than expected number of new hosts.
As computer models of infectious disease grew more powerful and precise, scientists began to realize that a lot of infectious disease spread is due to superspreaders. Researchers call it the 80/20 rule: Twenty percent of the population is responsible for 80 percent of disease spread.
The key to understanding and stopping outbreaks of infectious disease means homing in on this small portion of the population of superspreaders.
Accounting for the presence of superspreaders also means accounting for their much more common counterparts: people who don't spread the disease to anyone.
Taken together, that means a disease is much more likely to be introduced into new areas by superspreaders, but it is also more likely to fizzle out in those places due to nonspreaders.
Only occasionally, as with SARS, can the pathogen be introduced to somewhere new and then, like a spark on dry kindling, ignite a massive blaze of infection.
SARS traced to doctor
On Feb. 21, 2003, a 64-year-old doctor checked into Room 911 at the Hotel Metropole in Hong Kong. He was already sick with SARS but felt well enough to travel. Public health officials documented a total of seven people directly infected by the doctor, making him a superspreader.
Several of the people he infected were also superspreaders.
One of these infected individuals arrived in Toronto. Several days after she returned home, she developed symptoms consistent with SARS and ultimately died.
Nine other SARS cases and three deaths were directly linked to her, and the outbreak ultimately spread to 257 others. This chain of contagion accounted for more than 70 percent of Toronto's total SARS cases.
Although Toronto was besieged by SARS in 2003, Vancouver (with a similar population) saw only a handful of cases. The reason? No superspreaders were infected in Vancouver, said infectious disease expert James Lloyd-Smith of UCLA.
Superspreaders are not limited to humans.
When researchers began to deconstruct the outbreaks of West Nile virus that hit the United States, they found that certain areas were hit especially hard.
Robins spread west nile
Tony Goldberg, an epidemiologist at the University of Wisconsin-Madison, began to study the mosquitoes that transmitted West Nile virus to figure out why some places had so many more cases than others.
Goldberg and colleagues collected mosquitoes from around Chicago and found that, overwhelmingly, the mosquitoes had been feeding on the American robin.
West Nile virus is traditionally associated with crows, blue jays and other corvids, Goldberg said. These birds get sick and die in large numbers during West Nile outbreaks. Public health officials knew to brace themselves for a West Nile outbreak in humans this summer when they began to see large numbers of dead crows.
Although researchers knew that other birds could become infected, they were thought to be fairly minor contributors to West Nile spread.
The fact that robins don't die or become noticeably ill made them both invisible to public health authorities and remarkably good spreaders of West Nile.
When crows and related birds die, they are unable to transmit the virus to other birds. They become literal dead ends.
Robins, on the other hand, continue the everyday activities that bring them in contact with new mosquitoes, which drink the virus from the birds' blood and continue the spread of the disease.
