**Core Concept**
The question is testing the concept of the "disease attack rate" in epidemiology. A disease attack rate is a measure of the frequency of a disease in a defined population over a specific period.

**Why the Correct Answer is Right**
The correct answer is related to the concept of "prevalence," which refers to the total number of cases of a disease in a population at a given time. Prevalence is often expressed as a percentage and takes into account both new and existing cases of the disease. In contrast, incidence refers to the number of new cases of a disease over a specific period.

**Why Each Wrong Option is Incorrect**
**Option A:** Incidence is the number of new cases of a disease over a specific period, whereas prevalence is the total number of cases (new and existing) in a population at a given time.

**Option B:** Morbidity rate refers to the number of cases of a disease per 1,000 people in a population, but it does not specifically address the excess number of cases in a community.

**Option C:** Mortality rate refers to the number of deaths from a disease per 1,000 people in a population, which is unrelated to the excess number of cases.

**Clinical Pearl / High-Yield Fact**
Prevalence is an essential measure in epidemiology as it helps in understanding the burden of a disease on a community and planning public health interventions.

**Correct Answer:** B.