**Core Concept**
Polio is a highly infectious disease caused by the poliovirus, and its transmission can lead to epidemics, which are critical public health events. The criteria for defining a polio epidemic are essential for implementing control measures and allocating resources.

**Why the Correct Answer is Right**
A polio epidemic is typically defined by an increase in the number of polio cases above the expected number, which can be determined by analyzing historical data or surveillance reports. This increase in cases can be due to various factors, such as a new virus strain, changes in human behavior, or environmental factors. The World Health Organization (WHO) and national health authorities use these criteria to determine if a polio outbreak has reached epidemic proportions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a recognized criterion for defining a polio epidemic. While the number of cases is an important factor, it is not the only criterion.
**Option B:** This option is incorrect because it is not a specific criterion, but rather a general statement about polio outbreaks. While polio outbreaks can occur in areas with low vaccination coverage, this is not a defining characteristic of an epidemic.
**Option C:** This option is incorrect because it is not a recognized criterion for defining a polio epidemic. While the geographical spread of cases can be an important factor in outbreak response, it is not a defining characteristic of an epidemic.

**Clinical Pearl / High-Yield Fact**
The key to controlling polio outbreaks is rapid detection and response, which involves collaborating with local health authorities, implementing vaccination campaigns, and improving sanitation and hygiene practices.

**Correct Answer: C. The geographical spread of cases is not a criterion for defining a polio epidemic.**