Boils under the armpit are usually caused by an infection of a hair follicle or oil gland. The infection, usually involving the bacterium Staphylococcus aureus, builds up in the follicle in the form of pus and dead skin, causing the area to become red and raised. Boils can occur due to excessive sweat, poor hygiene, shaving, or a preexisting condition. Hidradenitis suppurativa is a condition that can cause armpit boils and affects about 1-4% of the population. It is caused by blockage of the hair follicles, which become inflamed and swollen, and bacteria may move in and make the inflammation worse. Other factors that can increase the risk of developing boils include close contact with someone who has a staph infection, obesity, a skin disorder such as eczema, and a weakened immune system. If the boil under your arm grows rapidly or does not improve in two weeks, see your doctor. Your boil may need to be surgically lanced (opened by cutting a small incision) .