1. Evidence of recurrent or continuous inflammation ( localised or systemic) of otherwise undetermined cause, which fluctuate apparently randomly, either periodically or irregularly AND
2. Infectious and autoimmune testing will have been non-diagnostic.

Attacks typically start during childhood but symptoms can also begin during adolescence or even in later adulthood. Main symptom is fever. Other symptoms include serositis (peritonitis, pleuritis and pericarditis), recurrent stroke-like episodes, myalgia, arthralgia and rash, CNS, gastrointestinal and respiratory symptoms.