Pneumonia: This is often the most common cause of unilateral haziness on a chest X-ray. The left side can be affected by community-acquired pneumonia, which can be caused by bacteria, viruses, or fungi. Symptoms such as cough, fever, and difficulty breathing often accompany pneumonia.

Pulmonary Edema: This condition, which involves fluid accumulation in the lungs, can cause haziness on a chest X-ray. It can be unilateral, especially if it's related to a specific condition like mitral regurgitation affecting the left lung more.
Pleural Effusion: Fluid accumulation in the pleural space can cause haziness on the affected side. Symptoms can include shortness of breath and chest pain.
Lung Contusion: Trauma to the chest can cause a lung contusion, which appears as haziness due to blood leakage into lung tissue.

Pulmonary Embolism: Although it might not always present with typical X-ray findings, a large pulmonary embolism can cause areas of haziness or oligemia (Westermark sign). It's crucial to consider this diagnosis due to its high mortality rate if untreated.
Tumor: A lung tumor can cause localized haziness, especially if it's large or if there are associated complications like pneumonia or atelectasis.
Empyema: An infection of the pleural space can cause haziness and is a serious condition requiring prompt treatment.

Lymphangitic Carcinomatosis: This rare condition involves the spread of cancer to the lymphatic vessels of the lungs, causing a pattern of interstitial markings that can appear as haziness.
Eosinophilic Pneumonia: A rare condition characterized by an accumulation of eosinophils in the lungs, which can cause haziness on X-ray.
Chronic Eosinophilic Pneumonia: A rare condition that can cause recurrent episodes of pneumonia-like symptoms and X-ray findings, including unilateral haziness.