Scientists initially hypothesized that acne represented a disease of the skin's hair follicle, and occurred due to blockage of the pore by sebum. During the 1880s, bacteria were observed by microscopy in skin samples affected by acne and were regarded as the causal agents of comedones, sebum production, and ultimately acne.[164] During the mid-twentieth century, dermatologists realized that no single hypothesized factor (sebum, bacteria, or excess keratin) could completely explain the disease.[164] This led to the current understanding that acne could be explained by a sequence of related events, beginning with blockage of the skin follicle by excessive dead skin cells, followed by bacterial invasion of the hair follicle pore, changes in sebum production, and inflammation.[164]

A nodule is an abnormal tissue growth which can either develop just below the skin or anywhere within the skin’s three layers (the epidermis, dermis, and subcutaneous tissue). Nodules commonly form in regions such as the face, neck, armpits, and groin, although they can also develop on internal organs such as the lungs, thyroid, and lymph nodes. They create solid, raised lumps that are more than 1 to 2 centimeters in diameter, with the potential to reach up to the size of a hazelnut. Nodules are hard and firm to the touch, unlike cysts whose pus makes them softer to the touch. This type of severe acne should be consulted by a doctor, as it might be indicative of a more serious condition.

Diet. Studies indicate that certain dietary factors, including skim milk and carbohydrate-rich foods — such as bread, bagels and chips — may worsen acne. Chocolate has long been suspected of making acne worse. A small study of 14 men with acne showed that eating chocolate was related to a worsening of symptoms. Further study is needed to examine why this happens and whether people with acne would benefit from following specific dietary restrictions.