Anticipating the effects of a treatment on a patient, informing the decisions of anesthesiologists... Digital twins open up new perspectives in medicine and limit invasive procedures. Dominique Chapelle, Director of Research at Inria and researcher at the École Polytechnique's Solid Mechanics Laboratory (*LMS), is developing these “patient twins” using mathematical models based on the description of cause and effect in numerous phenomena. On World Health Day, April 7, 2025, we focus on these digital patients and their predictive capabilities.