**Core Concept**
Herd immunity refers to the protection of a community from infectious diseases when a sufficient percentage of individuals within the community are immunized, thereby preventing the spread of the disease. This concept is crucial in preventing the outbreak of infectious diseases, especially in vulnerable populations such as young children, the elderly, and those with compromised immune systems.

**Why the Correct Answer is Right**
Herd immunity is achieved when a sufficient percentage of a population is immunized, thereby preventing the spread of a disease. This can be calculated using the herd immunity threshold, which varies depending on the infectiousness of the disease. For example, the herd immunity threshold for measles is approximately 93-95%, meaning that if 93-95% of a population is immunized, the disease will not spread in the community. This is because the infectious individuals are less likely to come into contact with susceptible individuals, thereby preventing the spread of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Herd immunity can be achieved through vaccination of a sufficient percentage of the population. This statement is true, as vaccination is a primary means of achieving herd immunity.
**Option B:** Herd immunity can protect not only the immunized individuals but also the susceptible individuals in the community. This statement is also true, as herd immunity provides indirect protection to those who are not immunized.
**Option C:** Herd immunity can be achieved even if the vaccine is not 100% effective. This statement is true, as herd immunity can be achieved even if the vaccine is not perfectly effective, as long as a sufficient percentage of the population is immunized.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that herd immunity is not just about protecting the immunized individuals, but also about protecting the susceptible individuals in the community. This is why it is essential to maintain high vaccination rates in the community to prevent the spread of infectious diseases.

**Correct Answer: D.**