**Herd Immunity Core Concept**
Herd immunity is a phenomenon where a significant portion of a population becomes immune to a disease, thereby indirectly protecting those who are not immune, such as individuals with compromised immune systems or those who have not yet developed immunity. This concept relies on the idea that the spread of a disease is reduced when a sufficient percentage of the population is immune.

**Why the Correct Answer is Right**
Herd immunity can be achieved through vaccination, where a sufficient percentage of the population is vaccinated against a particular disease, thereby reducing the likelihood of disease transmission. The level of herd immunity required varies depending on the disease, but it is generally considered to be around 80-95% for most vaccine-preventable diseases. For example, measles is highly contagious and requires a high level of herd immunity, typically above 93-95%, to prevent outbreaks.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because herd immunity can indeed be achieved through vaccination, which is a well-established method of preventing the spread of infectious diseases.
**Option B:** This option is incorrect because the concept of herd immunity does not apply solely to vaccines, but also to natural infections, where individuals who have recovered from a disease can also contribute to herd immunity.
**Option C:** This option is incorrect because herd immunity is not solely dependent on the presence of a specific disease in the population; rather, it is influenced by various factors, including the percentage of the population that is immune, the contagiousness of the disease, and the effectiveness of the vaccine or natural immunity.

**Clinical Pearl / High-Yield Fact**
A key aspect of herd immunity is that it is not a fixed concept and can change over time due to various factors, such as changes in vaccination rates, the emergence of new strains of a disease, or the introduction of new vaccines.

**Correct Answer: A.**