During the COVID-19 pandemic in late 2020, testing kits for the viral infection were scant in some countries. So the idea of diagnosing infection with a medical technique that was already widespread — chest X-rays — sounded appealing. Although the human eye can’t reliably discern differences between infected and non-infected individuals, a team in India reported that artificial intelligence (AI) could do it, using machine learning to analyse a set of X-ray images. The paper — one of dozens of studies on the idea — has been cited more than 900 times. But the following September, computer scientists Sanchari Dhar and Lior Shamir at Kansas State University in Manhattan took a closer look. They trained a machine-learning algorithm on the same images, but used only blank background sections that showed no body parts at all. Yet their AI could still pick out COVID-19 cases at well above chance level.