There are different types of acne. The most common acne is the type that develops during the teen years. Puberty causes hormone levels to rise, especially testosterone. High hormones cause signal skin glands to start making more oil (sebum). Oil releases from the pores to protect the skin and keep it moist. Acne begins when oil mixes with dead cells and clogs the skin's pores. Bacteria can grow in this mixture, and if it leaks into nearby tissues, it causes swelling, redness, and pus. A common name for these raised bumps is pimples.

See an illustration of how pimples form.

Certain medications can cause acne to develop. This type of acne usually clears up when you stop taking the medication.

It isn't just teens who are affected by acne. Sometimes newborns have acne because their mothers pass hormones to them just before delivery. Acne can also appear when the stress of birth causes the baby's body to release hormones on its own. Young children and older adults also may get acne.

A few conditions of the endocrine system, such as polycystic ovary syndrome and Cushing's syndrome, can lead to outbreaks of acne.