Final answer:
The periods when a chronic illness is present but the patient does not experience symptoms are known as periods of remission.
Step-by-step explanation:
Chronic illnesses often have a slow onset and feature periods known as remission, where the disease is still present but the person does not experience symptoms. Chronic diseases are long-term health conditions that may persist for a lifetime, such as type 1 diabetes and cardiovascular diseases. In the context of infectious diseases, this occurs in cases like herpes and AIDS, where the pathogens are not fully eliminated and can persist in a dormant or less active phase. Thus, the correct option is (a) Remission. Other key terms related to the stages of disease include the incubation period, prodromal period, period of illness, period of decline, and period of convalescence.