What term describes a rapid onset of symptoms often used to indicate the severity of an illness?

The term that describes a rapid onset of symptoms is "acute." This terminology is often used in medical contexts to indicate that a condition has developed quickly and requires immediate attention. Acute conditions can manifest with severe symptoms that may arise suddenly, making it essential to address them promptly to prevent further complications.

In contrast to "acute," chronic refers to conditions that develop gradually and persist over a long period. Long-term suggests an extended duration without necessarily implying severity or sudden onset. Persistent generally describes symptoms that continue over time but does not specifically denote a rapid onset, which is what acute specifically addresses. Therefore, "acute" is the most accurate term to represent conditions that arise quickly and are often associated with severe symptoms.