Pneumocystis jirovecii pneumonia (PCP): This is a common cause of bilateral pneumonia in neutropenic patients, especially those with HIV/AIDS or undergoing immunosuppressive therapy. The presentation of bilateral interstitial infiltrates in a neutropenic patient strongly suggests PCP.

Bacterial pneumonia: Bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and gram-negative rods can cause pneumonia in neutropenic patients. The risk is increased due to the compromised immune system.
Influenza and other viral pneumonias: Viral infections can lead to severe pneumonia in immunocompromised individuals, including those with neutropenia.
Fungal pneumonia: Aspergillosis, candidiasis, and other fungal infections are more common in neutropenic patients and can cause severe bilateral pneumonia.

Invasive aspergillosis: Although less common, invasive aspergillosis is a critical diagnosis to consider due to its high mortality rate in neutropenic patients. It can present with similar symptoms to bacterial or viral pneumonia but requires specific antifungal treatment.
Cytomegalovirus (CMV) pneumonia: CMV can reactivate in immunocompromised individuals, leading to severe pneumonia. It's particularly concerning in patients who have undergone organ or bone marrow transplants.

Mycobacterial infections: While less common, mycobacterial infections such as tuberculosis (TB) can occur in neutropenic patients, especially in endemic areas or in those with exposure history.
Nocardiosis: Nocardia species can cause pneumonia in immunocompromised patients, including those with neutropenia. It's a rare but important consideration due to its specific treatment requirements.
Pneumonia due to Legionella or other atypical bacteria: These pathogens can cause severe pneumonia in immunocompromised individuals, though they are less common in the context of neutropenia compared to other causes.