炎

Top fire 火 — pictograph of a flame: a central tongue rising between two side-sparks. 炎 is fire doubled, so the radical is core to the meaning. Sits above the second fire as the flame leaping up; the original sense was 'flames rising,' captured by the stacked picture.

Bottom fire 火 — the same pictograph repeated. Two fires stacked one above the other give the compound-ideograph meaning of 炎: blazing heat, raging flames, fierce sun. From there the modern medical sense 'inflammation' (肺炎 pneumonia, 发炎 to become inflamed) extends the heat metaphor into the body.