Pneumonia: Given the compromised immune status, pneumonia is a common and likely cause of fever due to the increased susceptibility to bacterial, viral, and fungal infections.

Urinary Tract Infection (UTI): Immunocompromised patients are at higher risk for UTIs, which can cause fever.
Central Line-Associated Bloodstream Infection (CLABSI): If the patient has a central line, this is a common source of infection in immunocompromised patients.
Clostridioides difficile (C. diff) Infection: Immunocompromised patients, especially those on antibiotics, are at increased risk for C. diff infection, which can cause fever.
Sinusitis: Given the impaired immune response, sinusitis can be a cause of fever in these patients.

Neutropenic Sepsis: A life-threatening condition that requires immediate attention, characterized by fever in the setting of neutropenia.
Invasive Fungal Infections: Such as aspergillosis or candidiasis, which can be deadly if not promptly diagnosed and treated.
Pneumocystis jirovecii Pneumonia (PCP): A serious and potentially life-threatening infection in immunocompromised patients, particularly those with HIV/AIDS or on immunosuppressive therapy.
Cytomegalovirus (CMV) Infection: Can cause severe disease in immunocompromised patients, including pneumonia, colitis, and retinitis.

Lymphoma: Certain types of lymphoma can present with fever of unknown origin in immunocompromised patients.
Tuberculosis (TB): Although less common in some regions, TB can cause fever in immunocompromised patients and is important to consider, especially in those from endemic areas.
Endocarditis: Infective endocarditis is a rare but serious condition that can cause fever in immunocompromised patients, particularly those with a history of intravenous drug use or prosthetic heart valves.
Visceral Leishmaniasis: A rare cause of fever in immunocompromised patients, typically seen in those who have traveled to or are from endemic areas.