Which term describes a condition that is new and usually of rapid onset?

The term that describes a condition that is new and usually of rapid onset is 'acute.' In medical terminology, 'acute' refers to diseases or symptoms that develop quickly, often severity being high and requiring immediate attention. This could apply to various situations, such as acute infections, which can manifest suddenly and cause significant health issues that may necessitate prompt treatment. It contrasts with chronic conditions, which develop slowly and persist over a long period.

While benign refers to a condition that is not harmful or cancerous, and terminal relates to a disease that is expected to lead to death, these terms do not specifically address the rapid onset of new conditions. Thus, 'acute' is the most appropriate choice to characterize a sudden medical issue.