CIRCLE is an ARPA-H program focused on improving care for critically ill patients by measuring and tracking changing immune responses, identifying actionable immune-system control points, and guiding interventions. Its planned approach combines immune-system data and diagnostics with AI-powered digital twins to help clinicians understand a patient’s evolving condition and evaluate possible treatments. ARPA-H selected research teams to develop measurement, modeling, and intervention capabilities, alongside shared data, testing, and clinical-trial support. The program aims to address critical illness associated with infection, injury, and chronic disease complications.