**Core Concept**
Viruses are obligate parasites that require a host cell to replicate and express their genetic material. They are acellular, meaning they do not have cellular structure, and their genetic material is either DNA or RNA, but not both.

**Why the Correct Answer is Right**
The correct answer to this question involves identifying a characteristic that is not true for all viruses. Viruses exhibit a range of properties, including their obligate parasitic nature, lack of cellular structure, and the presence of either DNA or RNA as genetic material. However, they do not have a cell wall, which is a characteristic of bacteria. Viruses can, however, contain proteins that form a capsid, which provides structural support and protects the genetic material.

**Why Each Wrong Option is Incorrect**
* **Option A:** Viruses do not have a cell wall, but they can have a capsid, which is a protein structure that surrounds the genetic material. This is a correct property of viruses.
* **Option B:** Viruses are obligate parasites, meaning they require a host cell to replicate and express their genetic material. This is a correct property of viruses.
* **Option C:** Viruses can have either DNA or RNA as their genetic material, but not both. This is a correct property of viruses.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that viruses do not have cellular structure and require a host cell to replicate. This distinction is crucial in differentiating viruses from other microorganisms, such as bacteria, which have a cell wall and can reproduce independently.

**Correct Answer: D. All viruses have a cell wall.**