See below. The disease starts with a plugging of the hair follicle followed by follicular rupture, and an associated immune response. Genetic susceptibility, mechanical stresses on the skin, obesity, smoking, diet, and hormonal factors are repeatedly cited as factors that may be associated with the development or exacerbation of Hidradenitis.See http://bit.ly/1yTavVt and http://bit.ly/1LKGM7Y.
Bacteria. Bacteria and friction! most people get it in areas that have a lot of exposure to friction like underarms and in the groin area. The friction from say, a razor, causes tiny tears in the skin allowing bacteria from the skin and possibly the razor to enter. Your body wants to wall off the infection and it mixes with cells that try to kill the bacteria. Leading to pain, swelling, pus and the boil.
Hidradenitis. Hidradenitis suppurativa (hi-drad-uh-NIE-tis sup-yoo-ruh-TIE-vuh) is rare, long-term skin condition that features small, painful lumps under the skin. They typically develop where the skin rubs together, such as the armpits, the groin, between the buttocks and under the breasts http://www.mayoclinic.org/diseases-conditions/hidradenitis-suppurativa/home/ovc-20200012.
H. suppurativa. Refer to my answer above for details. Usually cause unknown but tx is available. Read more...