Armpit boil is 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. The area will become red and raised, and will slowly grow as additional pus builds up within the lesion .
While unsightly and uncomfortable, most boils are not life-threatening and may open and drain on their own within two weeks . 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).
causes
A boil forms when a bacterial infection — most commonly a staph infection — occurs within a hair follicle. The infection affects the hair follicle and the tissue around it. The bacterial infection causes a hollow space around the follicle that fills with pus. If the area of infection increases around the hair follicle, the boil will grow larger.

Symptoms

Symptoms of a boil include:
red,
pinkish bump
pain on or around the bump
yellow pus showing through the skin
fever
sick feeling
itching on or around boil
Several interconnected boils are called a carbuncle . A carbuncle is a large area of infection under the skin. The infections result in a group of boils appearing as a larger bump on the surface of the skin.
Boils under the arm occur when a hair follicle becomes infected. This may occur due to:
Excessive sweating. [I] If you sweat more than normal due to the weather or physical activity, but you don’t clean yourself properly, you may be more susceptible to infections such as boils. [/I]

Shaving.
Your underarm is a place where sweat and dead skin can build up. If you shave your armpits often, you could have higher likelihood of contracting a bacterial infection in your armpit. When you shave, you may be accidentally creating openings in the skin under your arms which can allow bacteria easier access.

poor hygiene.

If you do not wash under your arms regularly, dead skin can build up which may contribute to the development of boils or pimples.
Weak immune system. If you have a weak immune system , your body may be less able to fight off a bacterial infection. Boils are also more common if you have diabetes mellitus, cancer , eczema or allergies .
Do not pick at, pop, or squeeze your boil. Among other negative results, popping your boil may cause the infection to spread. Also, squeezing the boil may allow additional bacteria to enter the lesion from your hands or fingers.

To help your boil heal:

Use antibacterial soap to clean the area.
Apply moist, warm compresses to the area several times a day.
Do not attempt to pop the boil.
If your boil does not go away after two weeks, you should get treatment from a medical provider. Your doctor may cut the boil open to drain the pus. You may also be prescribed antibiotic s to heal the underlying infection.
You may be wondering whether the bump in your skin under your arm is a boil or a pimple . A pimple is characterized by an infection of a sebaceous gland. This gland is closer to the top layer of the skin ( epidermis ) than a hair follicle. If a pimple is raised , it will likely be smaller than a boil.
A boil is an infection of the hair follicle which is located deeper in the second layer of skin (dermis), closer to the fat tissue beneath your skin. The infection then pushes out to the top layer of the skin creating a larger bump.
While uncomfortable, boils under your arm are not usually anything to worry about. The boil will most likely improve or heal itself within two weeks.
If you boil grows larger, sticks around for more than two weeks or causes you to have a fever or intense pain, talk to your doctor. You might need a prescription for antibiotics or your doctor might open and drain your boil.

mynd44,Seun, Lalasticlala.

Quite uncomfortable it is and also if it grows in your private region

Olalan:
Quite uncomfortable it is and also if it grows in your private region

Then allow it grow larger till it becomes an extra scrotum.

Isn't that a cheaper cure for your low-sperm count?

*spits