Acute: Of abrupt onset, in reference to a disease. Acute often also connotes an illness that is of short duration, rapidly progressive, and in need of urgent care.
"Acute" is a measure of the time scale of a disease and is in contrast to "subacute" and "chronic." "Subacute" indicates longer duration or less rapid change. "Chronic" indicates indefinite duration or virtually no change.
The time scale depends on the particular disease. For example, an acute myocardial infarction (heart attack) may last a week, while an acute sore throat may only last a day or two.