**Core Concept:** Herd immunity is a phenomenon where immunization of a large portion of a population prevents the disease from spreading among the unvaccinated individuals. In the case of diphtheria, immunity is achieved through the production of neutralizing antibodies against diphtheria toxin.

**Why the Correct Answer is Right:** A herd immunity threshold of 90% is typically considered necessary to prevent epidemic spread of diphtheria. This is because, on average, 10% of the population will have a temporary or permanent immunity due to previous infection or vaccination, thus protecting the remaining unvaccinated individuals.

**Why Each Wrong Option is Incorrect:**

A. 80%: This level of immunity may not be sufficient to prevent the spread of diphtheria, as it leaves a large portion of the population unprotected.

B. 95%: While 95% immunity may prevent diphtheria from spreading, it is not necessary for herd immunity threshold when considering the average immunity level.

C. 100%: Achieving complete immunity in a population is unlikely due to temporary immunity loss, and it is not the main goal of herd immunity.

D. 50%: This level of immunity does not provide sufficient protection for the unvaccinated individuals, allowing diphtheria to spread easily.

**Clinical Pearl:** Diphtheria is a serious bacterial disease causing respiratory and cardiovascular complications, and requires timely vaccination and boosters to maintain immunity in the population.

**Correct Answer:** 90%