**Core Concept**
Pigeon droppings can cause a specific type of pneumonia known as Histoplasmosis, which is a fungal infection caused by inhaling the spores of a fungus called Histoplasma capsulatum. This condition can lead to bilateral lung involvement and pneumonitis, as seen in the patient's presentation.

**Why the Correct Answer is Right**
Histoplasma capsulatum is a dimorphic fungus that grows as a mold in the environment, producing spores that can be inhaled. Once inhaled, the spores transform into yeast-like cells within the host's macrophages. The patient's history of exposure to pigeon faeces suggests that he may have inhaled the spores, leading to the development of Histoplasmosis. The presence of bilateral crepitations and pneumonitis is consistent with this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not relate to the patient's presentation or exposure history.
**Option B:** This option is incorrect as it refers to a different type of fungal infection, Coccidioidomycosis, which is typically associated with exposure to soil in certain regions of the southwestern United States.
**Option C:** This option is incorrect as it refers to a viral infection, Influenza, which is not typically associated with exposure to pigeon faeces.
**Option D:** This option is incorrect as it refers to a bacterial infection, Pneumocystis pneumonia, which is typically associated with immunocompromised individuals.

**Clinical Pearl / High-Yield Fact**
Histoplasmosis can mimic other conditions such as sarcoidosis, tuberculosis, or malignancies, and a high index of suspicion is necessary for diagnosis. A chest X-ray and serological tests can help confirm the diagnosis.

**Correct Answer:** C.