Violence spreads like a virus. If someone with Covid-19 coughs near you, the disease can enter your lungs. And if someone commits violence against you, or in your presence, violent tendencies can enter your brain.

That’s why violence, like a virus, often appears in clusters. One killing generates others, just as one person with Covid-19 can start a chain of infections. The best way to stop contagion is to interrupt it early: trace, isolate and prevent spread.

That was Gary Slutkin’s insight when he returned to the US in 1995 after 10 years in Africa combating tuberculosis, cholera and Aids. The epidemiologist and doctor now leads Cure Violence Global, an NGO. CVG employs “violence interrupters” who find people at risk of committing violence and deter them before they act.