A lesion in the gum comes and goes, and the associated tooth has no vitality. What is the cause?

A lesion (parulis/sinus tract opening) that intermittently appears and disappears over time, associated with a non-vital tooth, reflects a long-standing, low-grade infection that periodically drains and then temporarily closes — the hallmark of a chronic apical abscess. An acute abscess presents with a sudden, more continuously symptomatic and swollen course, not an intermittent on-and-off pattern, and pyogenic granuloma is unrelated to pulpal vitality.