Medical providers use the term "acute" to describe symptoms, a disease or a condition in one of three ways -- either when the problem comes on quickly, or if it lasts only a short time, or both.
Patients often think the word acute means severe, but professionals do not use the term in that way.
Examples are acute leukemia or acute appendicitis, both of which seem to develop quickly, and acute respiratory illness which may last only a short time.
Illnesses that last a long time are considered chronic.
The term "subacute" refers to a medical problem that is not exactly acute or chronic, rather somewhere in between.