Focussing in detail on one key component of the infodemic surrounding COVID-19, this article traces the dissemination dynamics of rumours that the pandemic outbreak was somehow related to the rollout of 5G mobile telephony technology in Wuhan and around the world. Drawing on a mixture of quantitative and qualitative methods including time-series analysis, network analysis and in-depth close reading, our analysis shows the dissemination of the rumour on Facebook from its obscure origins in pre-existing conspiracist groups through greater uptake in more diverse communities to substantial amplification by celebrities, sports stars and media outlets. The in-depth tracing of COVID-related mis- and disinformation across social networks offers important new insights into the dynamics of online information dissemination and points to opportunities to slow and stop the spread of false information, or at least to combat it more directly with accurate counterinformation.