Final answer:
The most common sign of an infectious disease of the nervous system is a headache, specifically associated with conditions like meningitis and encephalitis.
Step-by-step explanation:
The most common sign of an infectious disease of the nervous system is the presence of a fever. When the nervous system is infected, it triggers an immune response in the body, which often leads to an increase in body temperature.
Other signs and symptoms of an infectious disease of the nervous system may include headache, seizures, and tachycardia (fast heart rate), but fever is the most common and characteristic sign.