Acute refers to an illness or disease that begins suddenly and advances very rapidly. If an acute illness is not diagnosed quickly, it can become a major problem. It is the opposite of chronic, which means long and drawn out.
One of the key features that defines an acute illness is that it is very short in nature. Most often acute illnesses happen out of the blue with little forewarning. An example of an acute illness would be a heart attack, because everything can seem normal before the event occurs. The length of an acute illness depends on the specific medical problem, but for most, it is between a couple of days and a week or more.