Sebaceous glands in the skin produce sebum, which is an oily substance. When excessive sebum is produced, it mixes with dead skin cells to form a sticky mixture that clogs the hair follicles and pores. The trapped sebum and dead cells within the clogged pore develop an ideal environment for producing acne-causing bacteria. These bacteria can multiply and lead to inflammation of the pores. Thus, a red and swelling acne is formed. The following are common causes of acne: Excess Sebum Production Clogged Pores Hormonal Changes Bacterial Infection Genetic Factors Certain Medications  Dietary Factors Individuals’ lifestyle