Inflammation is a tier 3 symptom found in the standard plagues types and the Black Death scenario, and it is a tier 2 symptom in the Smallpox scenario. It is also present in the Mad Cow Disease scenario. It obstructs bodily processes, which can be fatal if breathing is obstructed.
Along with Fever, Tumours, and Diarrhoea, Inflammation is one of the four symptoms which can be better understood by scientists during a game. Having Inflammation evolved when this happens makes a plague easier to cure.