An acute illness is characterized by its sudden onset and typically short duration. This definition aligns perfectly with the answer that describes a sickness that begins quickly and lasts for a short time. Acute illnesses can often present with severe symptoms, but they usually resolve either on their own or with medical treatment within a relatively brief period.
For instance, conditions like the common cold, influenza, or a sprained ankle are considered acute because they start abruptly and most often do not persist beyond a few days or weeks. The correct answer captures the essence of what makes an illness acute—its brief and immediate nature, distinguishing it from chronic illnesses that develop gradually and last for longer periods.
The other options present different definitions that do not fit the criteria for acute illnesses. Long-term sickness with gradual onset describes chronic conditions, while progressive conditions needing ongoing management further characterize chronic illnesses. Lastly, an infection that spreads rapidly does not inherently define an acute illness, as the rapid nature of spread relates more to infectious diseases rather than the duration of the illness itself.