flare up

This expression conveys a sudden, explosive increase in intensity. Whether describing a physical fire, a medical symptom, or a social conflict, the core image is one of rapid escalation from a dormant or stable state to an active, volatile one.

In medical contexts, it specifically describes the transition from remission to an active phase of a chronic condition. In interpersonal or political contexts, it suggests a burst of anger or violence that often stems from long-standing, simmering tensions rather than a completely new disagreement.