Breast Cancer: This is the most likely diagnosis given the location of the lymphadenopathy (axillary) and the suspicion of cancer. Axillary lymph nodes are a common site for metastasis from breast cancer, and superinfection could occur in the setting of a tumor causing skin or tissue compromise.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with lymphadenopathy, including in the axilla. Superinfection could complicate lymphoma due to immune system dysfunction.
Infectious Causes (e.g., Cat Scratch Disease, Tuberculosis): These can cause axillary lymphadenopathy and might be complicated by superinfection, especially if the primary infection compromises the skin or soft tissues.
Metastatic Cancer from Other Sites: Cancers from other parts of the body (e.g., lung, melanoma) can metastasize to axillary lymph nodes, though this is less common than breast cancer.

Tuberculosis: Although less common in some regions, TB can cause lymphadenopathy and is crucial to diagnose due to its public health implications and the need for specific treatment.
Sarcoma: Soft tissue sarcomas can present with a mass and lymphadenopathy, and while rare, missing this diagnosis could have significant consequences.
Lymph Node Abscess: An abscess within a lymph node could present similarly to a superinfection and requires drainage for resolution.

Kikuchi-Fujimoto Disease: A rare, self-limiting condition that causes lymphadenopathy, more commonly in the neck but can occur in the axilla.
Kimura Disease: A chronic inflammatory disorder that can cause lymphadenopathy and eosinophilia, more common in Asian males.
Castleman Disease: A rare disorder that can cause lymphadenopathy, which can be localized or multicentric, and may be associated with infection or malignancy.

Each of these diagnoses has a different approach to management and treatment, emphasizing the importance of a thorough diagnostic workup.