Lymphoma: This is a common cause of persistent fever and hepatosplenomegaly, particularly in the context of neoplastic conditions. Lymphoma can involve the liver and spleen, leading to their enlargement, and systemic symptoms like fever are frequent.

Chronic Lymphocytic Leukemia (CLL): CLL can cause hepatosplenomegaly due to infiltration of the liver and spleen by malignant cells. Fever can be a part of the clinical presentation, especially if there is associated infection or transformation.
Chronic Myeloid Leukemia (CML): CML often presents with hepatosplenomegaly due to the accumulation of mature and immature granulocytes in these organs. Fever can occur, particularly during the accelerated phase or blast crisis.
Infectious Mononucleosis: Caused by Epstein-Barr virus (EBV), this condition can lead to fever, hepatosplenomegaly, and lymphadenopathy. It's more common in younger individuals but can occur at any age.
Tuberculosis: Although more commonly associated with pulmonary symptoms, extrapulmonary tuberculosis can cause hepatosplenomegaly and persistent fever, especially in immunocompromised patients or those from endemic areas.

Sickle Cell Disease with Sepsis: Patients with sickle cell disease can develop splenic sequestration crisis, which may present with hepatosplenomegaly and fever. Sepsis is a life-threatening complication that requires immediate attention.
Endocarditis: Infective endocarditis can cause persistent fever and may lead to splenomegaly due to septic emboli. It's a critical diagnosis to consider due to its high morbidity and mortality if left untreated.
Brucellosis: This zoonotic infection can cause fever, hepatosplenomegaly, and lymphadenopathy. It's essential to consider in individuals with occupational exposure to animals or their products.

Histiocytosis: Rare conditions like Langerhans cell histiocytosis or Erdheim-Chester disease can cause hepatosplenomegaly and systemic symptoms, including fever.
Castleman Disease: A rare lymphoproliferative disorder that can present with fever, lymphadenopathy, and hepatosplenomegaly.
Kala-Azar (Visceral Leishmaniasis): This parasitic infection can cause prolonged fever, hepatosplenomegaly, and pancytopenia, particularly in endemic areas.
Mastocytosis: Systemic mastocytosis can lead to hepatosplenomegaly and may cause fever among other systemic symptoms due to mast cell mediator release.