Periodic fever, aphthous stomatitis, pharyngitis and adenitis syndrome [PFAPA] is a medical condition, typically starting in young children, in which high fever occurs periodically at intervals of about 3–5 weeks, frequently accompanied by aphthous-like ulcers, pharyngitis and/or cervical adenitis (cervical lymphadenopathy). The syndrome was described in 1987 and named two years later."

Source : Wikipedia

PFAPA is the most common periodic fever syndrome in children, but its cause(s) and its pathogenesis are currently unknown. Suggested possible causes include problematic genetics and/or effects of previous infections.

Research from 2020 suggests that Behçet's diseaseplugin-autotooltip__plain plugin-autotooltip_bigBehçet's disease

"Behçet’s Syndrome (also known as Behçet’s Disease or, simply, Behçet’s) is a chronic condition resulting from disturbances in the body’s immune system. This system, which normally protects the body against infections by producing controlled inflammation, becomes over-active and results in unpredictable outbreaks of unwanted and exaggerated inflammation. This extra inflammation particularly affects blood vessels of all sizes, including arteries and veins. As a result, symptoms… may be on a 'common spectrum' along with PFAPA - and both could be different manifestations of the same 'spectrum disorder'.

See: Common genetic susceptibility loci link PFAPA syndrome, Behçet’s disease, and recurrent aphthous stomatitis PNAS, 117 (25) 14405-14411