Pneumonia: This is the most likely diagnosis given the symptoms of fever and costophrenic pain, which can indicate an infection in the lower lobes of the lungs, causing inflammation that irritates the diaphragm and leads to pain that is referred to the costophrenic angle.

Pleurisy: Inflammation of the pleura, which can be caused by viral or bacterial infections, can lead to sharp chest pain that worsens with breathing, fever, and costophrenic pain.
Pulmonary Embolism: While not as common as pneumonia, pulmonary embolism can cause sudden onset of chest pain, fever, and can lead to costophrenic pain if the embolism affects the lower parts of the lung.
Costochondritis: Inflammation of the cartilage that connects the ribs to the breastbone can cause chest pain, but it less commonly presents with fever unless there's an infectious cause.

Tuberculosis (TB): Although less common in some regions, TB can present with chronic fever, weight loss, and costophrenic pain due to pleural involvement. Missing this diagnosis can have significant consequences.
Pneumocystis jirovecii Pneumonia (PCP): Especially in immunocompromised patients, PCP can cause severe pneumonia with fever, chest pain, and costophrenic pain, requiring prompt treatment.
Empyema: A collection of pus in the pleural space can cause fever, chest pain, and costophrenic pain. It's a serious condition that requires drainage and antibiotic treatment.

Lymphoma: Certain types of lymphoma can cause fever and chest pain, including costophrenic pain, due to mediastinal or pleural involvement.
Sarcoidosis: This autoimmune disease can cause inflammation in various parts of the body, including the lungs, leading to symptoms like fever and chest pain.
Ehlers-Danlos Syndrome: A genetic disorder that can lead to spontaneous pneumothorax, which might present with sudden chest pain and could potentially involve the costophrenic area.