Heat illness - what's wrong with body thermostat?

Depends. All non-heat-stroke issues stem from "local" (usually environmental) conditions causing too much heat in / produced / retained for body's propper fxn. Breathable fabrics, good hydration, exposure limits can all reduce these risks. Heat stroke esentially is failure of body's thermostat--often d/t prolonged issues above, some occur quick w/temp extremes--& can be fatal if not cooled rapidly.