Conspiracy theories (evidence-free, improbable narratives about powerful agents conspiring to harm people) circulated widely during the acquired immune deficiency syndrome (AIDS) and COVID-19 epidemics. They travelled over time and space, adapted to local conditions and anxieties, and were promoted for commercial and political purposes. Russian conspiracy theories claiming the viruses were United States bioweapons appeared in both epidemics. So did 'cultropreneurs' who, as a marketing strategy for their 'alternative' therapies, promoted conspiracy theories about scientific medicine. Pro-science activists sought to counter medical misinformation and debunk faux cures, but their task was harder in the 'post-truth' social-media driven context of COVID-19. Conspiracy theories about vaccines are an ongoing challenge for public health.
Keywords: AIDS; COVID-19; conspiracy theory; medical populism; misinformation.