A 55-year-old man with a smoking and drinking history presents with cough and flu-like symptoms. The silver stain of sputum shows gram negative rods. What is the likely diagnosis?

The presentation of a 55-year-old man with a significant smoking and drinking history, accompanied by cough and flu-like symptoms, points towards a respiratory infection. The identification of gram-negative rods in the sputum using a silver stain is a key diagnostic clue.

Legionella pneumonia, caused by Legionella pneumophila, is known for its association with specific risk factors, including smoking and drinking, as they can impair immune responses and respiratory function. The use of silver stain is crucial in identifying Legionella since it is not typically visible on standard gram staining. In clinical settings, Legionella pneumonia often presents with atypical pneumonia symptoms, including cough, fever, myalgias, and occasionally gastrointestinal symptoms, which align with the flu-like symptoms described.

Given the clinical context, the presence of gram-negative rods in the sputum, and the specific staining characteristics of Legionella, the diagnosis of Legionella pneumonia is strongly supported in this case.