During puberty or during adolescence, male androgenic hormone-testosterone is produced in both sexes. testosterone gets converted into DHT (di-hydro testosterone ) which stimulates the oil glands, to produce sebum which is required for the skin to be lubricated and impart lustre. Dust in our surroundings is attracted due to this fat (sebum), aiding the growth of bacteria. This bacteria enters the pores and breaks up this fat to fatty acid which causes irritation. In addition there is a lot of skin-cell (epithelial ) debris. this creates a block at the neck of the gland causing a bulge. this becomes infectious and also results in pus formation. this can spread to adjacent glands and become bigger in size.