System 100 comprises a contact surface, possibly an adhesive skin patch, having a sensor 104 in thermal contact 120 with the contact surface and a radio-frequency identification (RFID) chip 110. The system responds to induced signals from a reader, by transmitting e.g. human or animal core temperature via e.g. the RFID chip. Further detectors including position, chemical or pressure sensors may be used. An antenna 130 is located either separated from or is on the contact surface, being designed to operate on skin with correct resonant frequency, and a metallic reflector may be used. The organism core temperature 520 may be calculated by modelling heat flow, using the difference between sensor 104 and an additional sensor 106 across an insulating material 122 with known heat transfer coefficient. Ambient temperature may be compensated for. The information combined with user location could track spread of infection, mapping to known fever patterns.
