When bacteria enters the hair follicle and causes infection and inflammation, a furuncle or boil may develop. This is a pus-filled bump that appears on your skin. A collection of boils is called a carbuncle.
The face, the back of the neck, armpits, thighs, and buttocks are the areas most likely to be affected by boils (furuncles), which typically begin as reddish or purplish, tender bumps that quickly fill with pus and grow larger and more painful until they rupture and drain. You can usually treat a single boil at home, but avoid poking or squeezing it as this could spread the infection.