The technology provides an external device for estimating a core body temperature of a human subject. The wrist monitor device includes a casing sized and shaped to fit onto a wrist of a human subject. The casing has a first face and a second face. In addition there is a cavity in the casing that houses a heat trap. The heat trap has an opening oriented to receive incoming thermal energy from the skin and a shell of a thermal-insulation material that has an inner thermal energy-reflective surface. Detected temperature data is compiled in a database and used to estimate whether an identified individual has a core body temperature indicative of a fever.