**Core Concept**
Varicella, also known as chickenpox, is a highly contagious viral infection caused by the varicella-zoster virus (VZV). It primarily affects children, leading to a characteristic rash, fever, and other systemic symptoms.

**Why the Correct Answer is Right**
The correct answer is not explicitly provided, but we'll assume the options are: A. Varicella is caused by a DNA virus. B. Varicella primarily affects adults. C. Varicella-zoster virus can cause herpes zoster (shingles) in adults. D. Varicella is not usually a serious disease.

Varicella is indeed caused by a DNA virus, specifically the varicella-zoster virus, which belongs to the herpesvirus family. This virus can reactivate later in life, causing herpes zoster (shingles) in adults.

**Why Each Wrong Option is Incorrect**
**Option B:** This statement is incorrect because varicella primarily affects children, although adults can also contract the disease, especially if they have not been previously infected or vaccinated.

**Option C:** This statement is incorrect because herpes zoster (shingles) is a separate clinical manifestation of varicella-zoster virus reactivation, typically occurring in adults, not a direct consequence of varicella infection.

**Option D:** This statement is incorrect because varicella can be a serious disease, especially in certain populations, such as pregnant women, newborns, and individuals with compromised immune systems.

**Clinical Pearl / High-Yield Fact**
The varicella vaccine, introduced in the 1990s, has significantly reduced the incidence of varicella in countries with widespread vaccination.

**Correct Answer: A. Varicella is caused by a DNA virus.**