**Core Concept**
The secondary attack rate is a crucial measure used in epidemiology to assess the communicability of a disease within a population. It is defined as the number of new cases of a disease that occur among the contacts of a primary case within a specified time period, divided by the total number of exposed contacts. This measure helps public health officials understand the contagiousness of a disease and inform control measures.

**Why the Correct Answer is Right**
The secondary attack rate is a direct measure of the communicability of a disease because it quantifies the proportion of exposed contacts who develop the disease. This rate is influenced by factors such as the transmissibility of the disease, the duration of infectiousness, and the effectiveness of control measures. For example, a high secondary attack rate indicates that a disease is highly contagious, while a low rate suggests that the disease is less likely to spread.

**Why Each Wrong Option is Incorrect**
**Option A:** Case fatality rate refers to the proportion of individuals who die from a disease among those diagnosed with it, not the communicability of the disease.

**Option C:** Sullivan index, also known as the Disability-Free Life Expectancy (DFLE), is a measure of the average number of years a person is expected to live free from disability, not a measure of communicability.

**Option D:** Incubation period is the time between exposure to a pathogen and the onset of symptoms, which does not directly measure the communicability of a disease.

**Clinical Pearl / High-Yield Fact**
When assessing the communicability of a disease, it's essential to consider the secondary attack rate, as well as other factors such as the basic reproduction number (R0) and the serial interval. Understanding these measures can inform the development of effective control measures and public health interventions.

**✓ Correct Answer: B. Secondary attack rate**