**Core Concept:** Herd immunity is a phenomenon that occurs when a significant proportion of a population becomes immune to a contagious disease, either through vaccination or previous infection. This reduces the spread of the disease among the unvaccinated population members. Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, which primarily affects the respiratory tract and can lead to serious complications if not treated promptly.

**Why the Correct Answer is Right:** Achieving herd immunity for diphtheria is crucial to prevent its epidemic spread. The threshold for herd immunity varies among pathogens, but for diphtheria, it is generally considered to be around 90-95%. This means that approximately 90-95% of the population needs to be immune (either vaccinated or infected) to prevent the disease from spreading among the remaining unvaccinated individuals.

**Why Each Wrong Option is Incorrect:**

A. 80%: This is lower than the recommended herd immunity threshold for diphtheria, so it is insufficient to prevent epidemic spread.
B. 100%: While achieving full vaccination coverage is ideal, it is not necessary to prevent epidemic spread, as herd immunity is maintained as long as the threshold (90-95%) is reached.
C. 20%: This is a very low percentage, which would not provide adequate protection for the unvaccinated population, making it unsuitable as a threshold for preventing diphtheria epidemic spread.
D. 70%: This falls short of the recommended herd immunity threshold for diphtheria, indicating that this percentage would not be sufficient to prevent epidemic spread.

**Clinical Pearl:** The concept of herd immunity is essential in public health strategies to control infectious diseases like diphtheria. Ensuring a high vaccination rate in a population helps protect not only vaccinated individuals but also those who cannot be vaccinated, such as infants, immunocompromised individuals, and those with underlying medical conditions. This collective immunity helps maintain a low prevalence of infectious diseases, reducing the risk of epidemic outbreaks.

**Correct Answer:** D. 70%

Herd immunity threshold for diphtheria is around 90-95%. Given the correct answer (70%), it is clear that this percentage is lower than the recommended threshold, making it unsuitable for preventing diphtheria epidemic spread.