**Core Concept**
Severe combined immunodeficiency disease (SCID) is a group of rare, genetic disorders characterized by severe impairment of both T-cell and B-cell immune functions. This results in increased susceptibility to infections and decreased ability to fight off pathogens.

**Why the Correct Answer is Right**
SCID is caused by mutations in various genes that are crucial for the development and function of immune cells. The most common form of SCID is caused by mutations in the recombination activating gene 1 (RAG1) or recombination activating gene 2 (RAG2), which are essential for the process of V(D)J recombination, a critical step in the maturation of T and B cells. Without functional RAG proteins, these cells are unable to undergo the necessary rearrangements of their antigen receptors, leading to severe immunodeficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because SCID is primarily a disorder of lymphoid cells, not myeloid cells. Myeloid cells, such as neutrophils and macrophages, are not significantly affected in SCID.
**Option B:** This statement is incorrect because SCID is characterized by decreased antibody production due to impaired B-cell function, not increased antibody production.
**Option C:** This statement is incorrect because SCID is a genetic disorder, not a viral infection.

**Clinical Pearl / High-Yield Fact**
A key aspect of SCID is that it often presents in the first few months of life with recurrent, severe infections. Patients with SCID may also exhibit failure to thrive, delayed growth, and developmental delays.

**Correct Answer:** C. SCID is a genetic disorder, not a viral infection.