More than 11,000 Americans are killed in assaults involving guns annually, and at least 50,000 more are injured. Among people between the ages of 15 and 24, nine of every 100,000 lives end due to a gun homicide. About 65 of every 100,000 people in this age group are injured by gun assaults every year.
…Diseases that spread through social connections follow a different pattern from airborne colds or food-borne illnesses, in which contagion among strangers is more common. Papachristos postulated that subjecting gun violence to a traditional epidemiologic study would reveal whether it indeed behaves like a blood-borne pathogen.
…Having identified a network of gun violence, the researchers used a mathematical model to examine whether the spread adhered to known patterns of social contagion. …The model assumed that closeness mattered—“You’re more likely to influence a friend than a friend of a friend,” says Green—and that exposure risk, meaning a gun violence victim infecting someone else, diminished over time.
…The Chicago shootings included in the study occurred, on average, every 85 days, lending further support to the notion of gun violence as an infectious disease. Those 85 days suggest an incubation period, the time between a host becoming infected and a symptom appearing. Repeat gunshot events often occur within hours, says Gary Slutkin, infectious disease physician at the University of Illinois and adviser to the World Health Organization, who was among the first public health experts to see violence as a pathogen. But the short time frame in which people moved from being a friend of a new victim to a victim themselves—just a few months—was surprising.