When Ebola ripped through communities in West Africa between 2014 and 2016, Tolbert Nyenswah saw at first hand how health workers extinguished the epidemic by finding and quarantining contacts of those who caught the disease. The former director of Liberia’s public-health institute thought contact-tracers would again rise to the challenge this year, keeping COVID-19 in check as it swept the globe. “Contact-tracing is one of the greatest tools that countries should deploy and use effectively to contain the outbreak,” he says.