**Core Concept**
The Sullivan index is a measure used in public health to assess the health expectancy of a population, taking into account both mortality and disability. It is a composite indicator that provides an estimate of the average number of healthy years that a person can expect to live, free from disability.

**Why the Correct Answer is Right**
The Sullivan index is calculated by subtracting the years lived with a disability from the total life expectancy at birth. This allows for a more comprehensive understanding of the health status of a population, beyond just mortality rates. The index is named after its developer, Dr. John R. Sullivan, who first introduced this concept in the 1960s. The calculation involves estimating the proportion of life spent in different health states, including those with disability.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Sullivan index does not specifically focus on infant mortality rates.
**Option B:** This option is incorrect because the Disability-Adjusted Life Years (DALYs) measure is a related concept, but it is not the same as the Sullivan index.
**Option C:** This option is incorrect because the Healthy Life Expectancy (HLE) measure is another related concept, but it is not the same as the Sullivan index.

**Clinical Pearl / High-Yield Fact**
The Sullivan index is an essential tool for public health policymakers and researchers to assess the effectiveness of health interventions and track progress towards improving population health.

**Correct Answer:** C.