Final answer:
The most severe stage of an infectious disease is the period of illness, where signs and symptoms are most obvious and severe.
Step-by-step explanation:
The period of illness is the stage during which the number of pathogens present in the host is greatest and the signs and symptoms of the disease are the most severe. It is during this period that the infectious disease has the most significant impact on the patient's health. The illness period is followed by the decline period, where the number of pathogen particles begins to decrease, and there is a reduction in the severity of signs and symptoms. However, during this period, patients can be susceptible to secondary infections because their immune systems may have been weakened by the primary infection. Ultimately, the patient enters the period of convalescence, where they generally return to normal functions, though some diseases may cause permanent damage.
In contrast, the incubation period is the initial phase after the pathogen enters the host and begins multiplying without causing notable signs and symptoms. The prodromal period, which comes after incubation, is when the host begins to experience general signs and symptoms. During the convalescence period, the patient recovers and symptoms subside.
To answer the original question, the period of illness (C) is the most severe stage of an infectious disease.