We can classify a disease for example according to the severity (mild, intermediate, and severe) or according to the stability (stable and non-stable) and so on. According to what we determine acute and chronic?
Usually they are classified under the phases or stages of a disease.
According to the phase or stage of disease: 1. Acute. 2. Chronic.
See also here: stage of the condition (https://www.physio-pedia.com/Stage_of_the_Condition)