Bacillus Calmette–Guérin (BCG) is the most widely used vaccine worldwide and has been used to prevent tuberculosis for a century. BCG also stimulates an anti-tumour immune response, which urologists have harnessed for the treatment of non-muscle-invasive bladder cancer. A growing body of evidence indicates that BCG offers protection against various non-mycobacterial and viral infections. The non-specific effects of BCG occur via the induction of trained immunity and form the basis for the hypothesis that BCG vaccination could be used to protect against the severity of coronavirus disease 2019 (COVID-19). This Perspective article highlights key milestones in the 100-year history of BCG and projects its potential role in the COVID-19 pandemic. Bacillus Calmette–Guérin (BCG) has been used to prevent tuberculosis for a century and is also a standard approach for the treatment of non-muscle-invasive bladder cancer. However, BCG also has a plethora of non-specific effects that occur via the induction of trained immunity and have raised the hypothesis that BCG vaccination could be used to protect against coronavirus disease 2019 (COVID-19). In this Perspective, the authors describe the history of BCG, discuss the mechanisms of its effects, and consider its potential role during the COVID-19 pandemic.