**Core Concept**
Pityriasis rosea is a common skin condition characterized by an initial herald patch followed by a secondary eruption of oval-shaped, scaly patches. The exact etiology of pityriasis rosea is not fully understood, but it is believed to be related to a viral infection.

**Why the Correct Answer is Right**
The most widely accepted theory is that pityriasis rosea is caused by human herpesvirus 6 (HHV-6) or human herpesvirus 7 (HHV-7). These viruses are believed to trigger an immune response, leading to the characteristic skin lesions. The initial herald patch is thought to be a focal area of immune activation, which then spreads to other areas of the skin, resulting in the secondary eruption.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because pityriasis rosea is not caused by a bacterial infection. While some skin conditions are caused by bacteria, pityriasis rosea is a viral infection.
* **Option B:** This option is incorrect because pityriasis rosea is not caused by a fungal infection. While some skin conditions are caused by fungi, pityriasis rosea is a viral infection.
* **Option C:** This option is incorrect because pityriasis rosea is not caused by a parasitic infection. While some skin conditions are caused by parasites, pityriasis rosea is a viral infection.

**Clinical Pearl / High-Yield Fact**
The herald patch of pityriasis rosea is often located on the back or chest and can be mistaken for a ringworm or eczema. However, the characteristic oval-shaped, scaly patches that follow are more typical of a viral infection.

**Correct Answer: D. Human herpesvirus 6 (HHV-6) or human herpesvirus 7 (HHV-7).**