Your pores each contain an oil gland. The oil glands make an oily substance called sebum. Acne happens when these glands get plugged with sebum, dead skin cells, and dirt. The P. acnes bacteria that are normally found in the oil glands then multiply, causing inflammation. Acne is commonly triggered by changes in your hormones. These hormonal changes can cause the oil glands to get bigger and to make more sebum. Factors that can make acne worse include