Lymphadenopathy due to infection: This is often the most common cause of bilateral armpit lumps, resulting from infections such as the common cold, flu, or other viral and bacterial infections that cause the lymph nodes to swell.

Axillary lymphadenitis: Inflammation of the lymph nodes in the armpit, which can be caused by bacterial or fungal infections.
Cat scratch disease: Caused by the bacterium Bartonella henselae, transmitted through scratches from infected cats, leading to lymphadenopathy.
Hidradenitis suppurativa: A chronic skin condition characterized by recurring, painful abscesses and nodules in the apocrine-gland-bearing areas of the body, such as the armpits.

Lymphoma: A type of cancer that begins in cells of the lymph system, which can cause lymph nodes to swell in the armpits.
Breast cancer: Especially in women, as it can metastasize to the axillary lymph nodes, presenting as lumps in the armpits.
Tuberculosis: A bacterial infection that can cause lymphadenopathy, among other symptoms, and is crucial not to miss due to its public health implications and the need for specific treatment.

Sarcoidosis: A disease characterized by the growth of tiny collections of inflammatory cells (granulomas) in any part of the body, which can cause lymphadenopathy.
Silicone implant-associated lymphadenopathy: In individuals with silicone breast implants, there can be a rare reaction causing lymph nodes in the armpit to swell.
Kikuchi-Fujimoto disease: A rare, self-limiting condition characterized by lymphadenopathy, usually in the neck but can also occur in the armpits, and is more common in young women.