Tuberculosis (TB): Elevated monocytes can be seen in chronic infections like TB, where the body's immune response involves the activation and proliferation of monocytes. Low lymphocytes (lymphopenia) can also occur, especially in advanced or disseminated TB. Fever is a common symptom.

Bacterial Endocarditis: This condition can cause an increase in monocytes as part of the body's response to the bacterial infection. Lymphopenia may occur due to the immune system's response, and fever is a hallmark symptom.
Chronic Infections (e.g., Osteomyelitis, Abscesses): Chronic bacterial infections can lead to an increase in monocytes. Lymphopenia can be seen due to prolonged infection and inflammation, and fever is a common symptom.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause an elevation in monocytes due to chronic inflammation. Lymphopenia may occur, and fever can be present, especially during active disease phases.

HIV Infection: Although not the most common cause, HIV infection can lead to an elevation in monocytes and a decrease in lymphocytes (notably CD4+ T cells). Fever can be an early symptom. Missing this diagnosis could have significant implications for treatment and prognosis.
Lymphoma: Certain types of lymphoma can cause lymphopenia, and some may lead to an increase in monocytes. Fever can be a symptom, especially in more aggressive forms. Early diagnosis is crucial for effective treatment.
Sepsis: While sepsis typically presents with an increase in neutrophils, some cases can have a monocytosis. Lymphopenia can occur, and fever is a defining characteristic. Sepsis is a medical emergency, making it a "do not miss" diagnosis.

Histiocytosis: A group of rare diseases involving abnormal proliferation of histiocytes (including monocytes). Some forms, like Langerhans cell histiocytosis, can present with fever, elevated monocytes, and lymphopenia.
Leukemia (e.g., Chronic Myelomonocytic Leukemia): Certain leukemias can cause an increase in monocytes. While lymphopenia and fever can be present, these conditions are less common and would typically be diagnosed through specific blood tests and bone marrow biopsies.
Whipple's Disease: A rare, systemic bacterial infection that can cause fever, weight loss, and malabsorption. An increase in monocytes and lymphopenia can be seen, but the disease is very rare and diagnosis often requires intestinal biopsy.