Internal and external causes: Viral and bacterial infections (boils, acne, etc...), inflammation of the subcutaneous fatty layer.
Preventative steps to take: Avoid manipulating your pimples, trying to pierce them or trying to extract their contents at the risk of causing an infection or even a spread.
How to reduce or eliminate: Consult a dermatologist; prescribe antibiotics to treat the infection or superinfection; perform surgery under local anesthesia to remove it; perform corticosteroid injections.
Definition: A well-defined lump, usually round in shape, greater than 5 mm in diameter, which may be palpable and visible to the naked eye and which develops under the skin. Nodules may be soft, hard, with irregular or regular borders, accompanied by pain on touch or spontaneously in some cases, and red in color (inflammatory). They are most often found on the face and are associated with severe acne or rosacea. Nodules are mostly benign, however, in less common cases, they can be a sign of cancer.