Typically, a hair grows from the hair follicle, upwards and onto the skin's surface. However, when it fails to break through the dermal barrier, and thus its growth occurs internally, it is referred to as ingrown. This results in the emergence of a small inflamed red bump, which can be itchy, at the location where the hair should have normally emerged.
Generally, an ingrown hair will disappear on its own, but in some cases, it continues to grow under the skin and tends to become infected, thereby causing a folliculitis and then an abscess. It thus becomes painful and fills with pus. If it has been scratched or fiddled with, scarring may even occur.
This phenomenon may be due to its curled shape, growth in the wrong direction or even due to excessively dry skin. They often appear after a waxing or shaving session. The skin type and the rubbing of tight clothing against the skin, which contributes to its thickening, can also lead to an ingrown hair.
Although they can appear on any part of the body where hair grows (excluding the palms of the hands, soles of the feet, etc...), some areas are more prone to developing them than others: in men, the most affected areas are the chin, chest and neck, whereas in women, it's the legs, underarms and pubic area.