What should you associate with the term 'rigor' in medical conditions?

The term 'rigor' in medical contexts specifically refers to shivering or a strong, involuntary response to a febrile illness, often a sign that the body is reacting to an infection. It indicates a sudden onset of chills and is typically associated with a fever, helping to regulate body temperature during illness. This physiological response is meant to increase the body's temperature to fight off pathogens.

In contrast, the other options relate to concepts that do not accurately represent the medical definition of rigor. Bursts of energy might suggest an active state, which is not associated with the involuntary reaction of shivering. Muscle relaxation indicates a calm or restful state, opposing the tension that accompanies rigor. Aggressive symptoms may involve more pronounced, severe manifestations of disease but do not encompass the specific involuntary action of shivering paired with fever. Thus, shivering in fever aptly captures the essence of what rigor signifies in a medical setting.