**Core Concept:**
The secondary attack rate of chickenpox refers to the proportion of susceptible individuals in a population who develop chickenpox after being exposed to an infected person. Understanding this concept is important for understanding the transmission dynamics of the disease and the effectiveness of vaccination programs. Chickenpox is caused by the varicella-zoster virus (VZV) and is highly contagious.

**Why the Correct Answer is Right:**
The correct answer is **D**: 10-20%. In a population with a high vaccination rate, the secondary attack rate would be lower due to reduced transmission. This is because vaccinated individuals cannot get chickenpox and thus, transmit the virus less frequently. In contrast, in a population with low vaccination rates, the secondary attack rate would be higher as more individuals are susceptible to infection and thus, transmit the virus more frequently.

**Why Each Wrong Option is Incorrect:**
A) An attack rate of 0% indicates that no susceptible individuals get infected, which is not true since some individuals may not have been exposed or have developed immunity through previous infection or vaccination.
B) An attack rate of 100% suggests that all susceptible individuals get infected, which is not realistic considering factors like age, immune status, and vaccination coverage.
C) An attack rate of 50% indicates that half of the susceptible individuals get infected, which is also not accurate due to the same reasons mentioned above.

**Clinical Pearl:**
Understanding the transmission dynamics of infectious diseases like chickenpox is crucial for public health strategies, such as vaccination programs and contact tracing. A higher vaccination rate leads to a lower secondary attack rate, reducing the overall burden of disease in a population. This concept helps healthcare professionals make informed decisions on vaccination policies and disease control measures.