Redness and swelling under the armpit with pain are considered to be caused by inflammation of the lymph nodes or sebaceous cysts secondary to infection. Inflammation of the lymph nodes is mainly caused by Staphylococcus aureus or Streptococcus haemolyticus infection, and usually requires oral antibiotics such as cefradine or levofloxacin hydrochloride, and the symptoms are usually controlled within a week or so. A sebaceous cyst is a mass formed after the opening of the sebaceous gland in the armpit has been blocked. It is hard in texture and has a tofu-like substance inside, and is particularly prone to abscess formation secondary to bacterial infection.