Viral Infections: Such as infectious mononucleosis (caused by Epstein-Barr virus), cytomegalovirus, or viral hepatitis. These infections are common and often present with lymphocytosis as a key laboratory finding.

Chronic Lymphocytic Leukemia (CLL): A type of cancer that affects the blood and bone marrow, characterized by the production of an excessive number of immature white blood cells, known as lymphocytes.
Lymphocytic Leukemia: Other forms of lymphocytic leukemia, such as acute lymphoblastic leukemia (ALL), can also cause an increase in lymphocyte count.
Tuberculosis: Although more commonly associated with a low lymphocyte count in advanced stages, early infection can sometimes present with lymphocytosis.
Hypersensitivity Reactions: Certain allergic reactions or drug reactions can lead to an increase in lymphocytes.

HIV Infection: Early stages of HIV infection can present with lymphocytosis, and missing this diagnosis can have significant implications for patient management and public health.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can cause an increase in lymphocyte count, and early detection is crucial for effective treatment.
Severe Infections: Certain severe bacterial infections, such as pertussis or Lyme disease, can cause lymphocytosis and require prompt treatment to prevent complications.

Castleman Disease: A rare disorder that involves an overgrowth of cells in the body's lymph nodes, which can lead to lymphocytosis.
Lymphoproliferative Disorders: Rare conditions such as lymphomatoid granulomatosis or post-transplant lymphoproliferative disorder can also cause an elevated lymphocyte count.
Autoimmune Disorders: Certain autoimmune diseases, such as rheumatoid arthritis or lupus, can sometimes present with lymphocytosis, although this is less common.