There’s a standard set of best practices for disease outbreaks that includes a process called contact tracing. Each time you identify someone infected, you figure out who they’ve been in proximity to during the time they were infectious. You then get the person infected to self-isolate, and also convince their contacts to do so, at least until they can be tested and found to be uninfected. Doing this successfully can bring the rate of infection down below the point where the outbreak is self-sustaining—even if not everybody’s picked up through contact tracing, there won’t be many who aren’t, and anyone they infect eventually will be.

For the COVID-19 pandemic, infection rates in many countries were initially so high that contact tracing was impractical. But a suite of social interventions—social distance,  mask wearing, limiting time out of the home, washing hands, etc.—were used to bring rates back down to where contact tracing could be effective again.