This patient mostly likely has postinfluenza pneumonia caused by Staphylococcus aureus. She presents with recurrent cough and dyspnea after recently being diagnosed with influenza pneumonia, and her chest radiograph shows cavitary lesions, which are most consistent with S. aureus. Secondary bacterial pneumonia following influenza is a significant cause of morbidity and mortality, particularly in patients aged 65 years or older. Influenza virus affects the tracheobronchial epithelium, impairing normal mucociliary clearance mechanisms and predisposing to bacterial infection. The most common clinical presentation in patients with secondary bacterial pneumonia is the exacerbation of respiratory symptoms and fever after a period of initial improvement of acute influenza symptoms. Patients with secondary bacterial pneumonia typically develop higher fevers than were present with influenza and a cough productive of purulent sputum. Chest radiography will show pulmonary infiltrates.

Legionella species are an uncommon cause of secondary pneumonia associated with influenza infection compared with Streptococcus pneumoniae and S. aureus. Additionally, this patient has no additional risk factors predisposing her to developing CAP from Legionella, such as exposure to a community outbreak or contaminated water source.

Pneumonia due to Pseudomonas aeruginosa is an uncommon cause of CAP, and generally occurs in patients with chronic lung disease. It would, therefore, be an unlikely cause of secondary pneumonia in this patient.

Although S. pneumoniae is more common than S. aureus in postinfluenza pneumonia, it does not typically produce the radiographic picture of cavitary lesions, so it is unlikely to be the causative pathogen in this patient.