Infective Endocarditis: This condition is characterized by infection of the heart valves, which can lead to murmurs due to valve dysfunction. Fever is a common symptom due to the infectious nature of the disease, and leukopenia can occur, especially if the infection is severe or if there is a significant inflammatory response. The combination of these symptoms, particularly in someone with a pre-existing heart condition or a history of intravenous drug use, makes infective endocarditis a leading consideration.

Acute Rheumatic Fever: This autoimmune complication of streptococcal infection can cause fever, murmurs due to valvulitis, and potentially leukopenia. It's a key consideration in areas where rheumatic fever is common.
Systemic Lupus Erythematosus (SLE): SLE can cause a wide range of symptoms, including fever, leukopenia, and cardiac manifestations such as murmurs due to Libman-Sacks endocarditis. The presence of other systemic symptoms would support this diagnosis.
Endocarditis due to Non-Bacterial Thrombotic Endocarditis (NBTE): Often associated with malignancies or sepsis, NBTE can cause murmurs and embolic phenomena. Fever and leukopenia could be present, especially in the context of an underlying malignancy or infection.

Prosthetic Valve Endocarditis: Although less common, this is a critical diagnosis to consider in patients with a history of valve replacement surgery, as it can be life-threatening and requires prompt treatment.
Toxic Shock Syndrome: While not directly causing murmurs, toxic shock syndrome can lead to fever, leukopenia, and cardiac dysfunction. It's a medical emergency that requires immediate recognition and treatment.
Disseminated Intravascular Coagulation (DIC): Associated with severe infections, malignancies, or trauma, DIC can cause a wide range of symptoms including fever, leukopenia, and potentially cardiac issues. It's a condition with high mortality if not promptly addressed.

Whipple's Disease: A rare, systemic bacterial infection that can cause fever, weight loss, and cardiac symptoms including murmurs. Leukopenia can also be present.
Cardiac Tuberculosis: Although rare in many parts of the world, cardiac involvement in tuberculosis can lead to a variety of symptoms including fever, leukopenia, and cardiac abnormalities such as murmurs.
Marantic Endocarditis: Associated with malignancies, this condition involves the formation of sterile vegetations on heart valves, which can lead to murmurs. Fever and leukopenia could be present, particularly in the context of an underlying cancer.