They are called superspreaders, the minority of people who are responsible for infecting many others during epidemics of infectious diseases. Perhaps the most famous superspreader was Typhoid Mary, presumed to have infected 51 people, three of whom died, between 1900 and 1907.
If superspreading had been completely controlled, almost two-thirds of the infections might have been prevented, scientists said.
More than 28,000 confirmed, probable and suspected cases of Ebola were reported in West Africa during the outbreak, including more than 11,000 deaths, according to the World Health Organization.
Researchers at Princeton University and Oregon State University conducted a retrospective analysis of the timing and location of 200 community burials between October 2014 and March 2015 in the urban areas around Freetown, the capital of Sierra Leone.
Using a mathematical model, they reconstructed the transmission network to see what proportion of cases were caused by superspreaders. They estimated that about 3 percent of the people infected were ultimately responsible for infecting about 61 percent of cases.
“It’s similar to looking at a blood spatter pattern and figuring out where the shooter was standing,” said Benjamin Dalziel, an assistant professor of population biology at Oregon State University and a co-author of the study.
Based on the evidence of disease transmission, that pattern — a small number of individuals responsible for the majority of infections — also holds true in Guinea and Liberia, the other two countries hit hardest by Ebola, Dalziel said.
“Superspreading was more important in driving the epidemic than we realized,” he said.
Superspreaders of Ebola tended to be children younger than 15 or adults between 40 and 55 years old, he said. They were based in the community rather than in health-care facilities, and they continued to spread the disease after many of the people first infected were already in treatment centers, where transmission was much better controlled.
Older adults were probably caring for the children. These caregivers were also more likely to be in charge of organizing large funerals, Dalziel said. Researchers said their findings about the importance of superspreaders were conservative because they focused only on people who had been buried safely.
Ebola spreads primarily through contact with bodily fluids. During the epidemic, superspreaders were cited in numerous news stories about Ebola’s spread. Often, transmission occurred during caregiving at home and during funeral preparations, especially washing and touching the bodies of loved ones.
Researchers said the study provided a new statistical framework that allowed scientists to measure how important superspreaders were in fueling the epidemic. In the future, public health officials should consider asking: “What are the scenarios where superspreading might occur?” Dalziel said. That question might help tailor better methods of controlling an outbreak, he said.
Superspreaders have played a role in the spread of several other infectious diseases. During the 2002-2003 outbreak of severe acute respiratory syndrome, or SARS, superspreading was considered one of the most notable features of the outbreak.
In one such event in Hong Kong, a 26-year-old man who was admitted to a hospital for treatment infected 156 people, including hospital staff, patients and visitors.
A single superspreader in a busy hospital emergency department also spread MERS, Middle East respiratory syndrome, to 82 people in just three days in May 2015 during an outbreak of the virus in South Korea.
The “index patient” was a 68-year-old man who had been to Bahrain, the United Arab Emirates, Saudi Arabia and Qatar before returning to South Korea.
MERS first emerged in humans in 2012 and has been spreading in Saudi Arabia and neighboring countries since then. It is caused by a coronavirus from the same family as SARS. The disease, in which patients develop acute respiratory illness with fever, coughing and breathing problems, has spread to 27 countries and killed up to 40 percent of those infected.