Infectious Mononucleosis: This condition, caused by Epstein-Barr virus (EBV), often presents with lymphadenopathy, including swollen lymph nodes in the axilla, along with fever, sore throat, and fatigue. The presence of bilateral axillary lymphadenopathy can be a clue, although it's not exclusive to this diagnosis.

Tuberculosis (TB): TB can cause lymphadenopathy, including in the axillary region. Although it's more commonly associated with pulmonary symptoms, extrapulmonary TB can manifest with lymph node swelling.
Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy. While it's less common for lymphoma to initially present with axillary lymph node swelling, it's a consideration, especially in the absence of infectious symptoms.
Cat-Scratch Disease: Caused by Bartonella henselae, this condition typically follows a cat scratch or bite and can lead to swollen lymph nodes, often in the axilla if the scratch was on the arm.
Brucellosis: This zoonotic infection can cause lymphadenopathy among other systemic symptoms. It's less common but should be considered in individuals with exposure to animals or their products.

Breast Cancer: Although less likely, axillary lymphadenopathy can be a sign of breast cancer, either as a primary presentation or as part of metastatic disease. Given the potential severity, it's crucial not to miss this diagnosis, especially in women.
Lymphoma (again, due to its potential severity and the importance of early diagnosis): While listed under other likely diagnoses, the severity and importance of not missing lymphoma warrant its mention here as well.
HIV Infection: Acute HIV infection can present with lymphadenopathy, among other symptoms. Identifying HIV early is critical for managing the disease and preventing long-term complications.

Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can lead to lymphadenopathy, including in the axilla.
Kikuchi-Fujimoto Disease: A rare, self-limiting condition that causes lymphadenopathy, typically in the neck but can also affect axillary nodes.
Sarcoidosis: Although more commonly associated with pulmonary symptoms and lymphadenopathy in the chest, sarcoidosis can rarely cause axillary lymph node swelling.
Silicone Implant Illness: In individuals with breast implants, especially those with silicone implants, axillary lymphadenopathy can be a symptom of an immune response to the implant material.