What is the most likely diagnosis for a patient with fever, cough, shortness of breath, and normal vital signs with nonspecific infiltrative changes on chest X-ray?

The symptoms presented—fever, cough, shortness of breath—combined with the findings of nonspecific infiltrative changes on chest X-ray—strongly suggest atypical pneumonia. This type of pneumonia is commonly caused by organisms such as Mycoplasma pneumoniae or Chlamydia pneumoniae, which often produce more subtle symptoms and radiographic changes compared to typical bacterial pneumonia.

Atypical pneumonia usually features a gradual onset of symptoms, including a non-productive cough and may occur in otherwise healthy individuals. The fact that the vital signs are normal may further indicate that the patient's respiratory distress is not as severe as what would be typically observed in more acute forms of pneumonia, like pneumococcal pneumonia, which often presents with high fever, chills, and more significant changes on physical examination.

In contrast, pneumococcal pneumonia is often associated with acute, severe symptoms and focal infiltrates on chest imaging, rather than the nonspecific patterns seen here. Acute bronchitis primarily presents with cough, may include some mild fever, but typically does not produce significant infiltrative changes on chest X-ray. Lastly, chronic obstructive pulmonary disease (COPD) is characterized by a history of long-term smoking or exposure to irritants and presents with chronic respiratory