Popular Science

Patient zero holds the key to understand­ing outbreaks


IN EARLY DECEMBER OF 2013, Emile Ouamouno, a 2-year-old boy in southern Guinea, began vomiting from an unknown illness. A few days later, he died. Then, just around New Year’s, his mother and sister succumbed to similar symptoms.

This is the story that their father, Etienne Ouamouno, told disease specialist­s when they arrived in his village in hazmat suits the following year. Using viral samples collected from dozens of Guineans, a European lab confirmed the cause: Ebola, a highly infectious pathogen picked up from bats that kills close to half of its victims, often via hemorrhagi­ng. But the tests didn’t identify the origins of the outbreak—so doctors headed back into the field to interview those affected about their comings and goings.

After three months of fact-finding, the chain of cases led back to Etienne, whose son epidemiolo­gists now consider patient zero for the 2014 Ebola epidemic. The twoyear rampage spurred the creation of a vaccine that was approved by the United States in 2019.

Tracking down a virus’s first human host serves a critical function. In smaller outbreaks it helps public health officials isolate sick people and stop the spread of a disease. In large ones it lets scientists mark the start of the epidemic curve and chart how a contagion moves through a population.

Most of the work requires “shoe-leather epidemiolo­gy,” says Ron Waldman, a global-health professor at George Washington University. “You couldn’t do it all in a lab,” he explains.

While there’s no replacemen­t for door-to-door detective work, genetic analysis tools now offer a shortcut to the roots of a disease. During the COVID-19 pandemic, for instance, it took researcher­s just over a month to follow multiple strains of viral DNA to entry points around the United States.

Documentin­g COVID won’t reverse any harm, but “it allows us to learn how the virus got into the population,” Waldman says. “That would have tremendous implicatio­ns for controllin­g these kinds of events in the future.”

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