**Core Concept**
Curshmann's crystals are a type of intracellular inclusion body found in the epithelial cells of the respiratory tract. They are composed of abnormal aggregates of keratin proteins that accumulate due to the disruption of the cytoskeleton.

**Why the Correct Answer is Right**
The formation of Curshmann's crystals is associated with viral infections, particularly respiratory syncytial virus (RSV) and influenza virus. The viral infection leads to the disruption of the host cell's cytoskeleton, causing the abnormal accumulation of keratin proteins. This, in turn, results in the formation of characteristic needle-like crystals that can be seen under the microscope. The presence of Curshmann's crystals is a diagnostic feature of certain viral infections, particularly in the context of respiratory illness.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Curshmann's crystals are not typically associated with bacterial infections.
**Option B:** This option is incorrect because Curshmann's crystals are not seen in fungal infections of the respiratory tract.
**Option C:** This option is incorrect because Curshmann's crystals are not a feature of parasitic infections.

**Clinical Pearl / High-Yield Fact**
Curshmann's crystals are often seen in conjunction with the presence of ground-glass cells, which are another type of abnormal epithelial cell that can be seen in viral infections.

**Correct Answer: C. Influenza virus infection.