An abscess is a painful, swollen pocket of pus that forms inside the body when it fights an infection. Think of it as your body's emergency response: when bacteria invade a cut or wound, white blood cells rush to the area to kill the germs. The battlefield of this fight, with dead bacteria, dead white blood cells, and tissue debris, becomes pus. Your body walls off this mess with a membrane, creating a swollen, tender bump.

Abscesses can form anywhere: on your skin, in your gums, or even deep inside organs. A common one is a tooth abscess, which happens when bacteria reach the soft pulp inside a tooth, usually through a cavity. The infection creates pressure and severe pain until a dentist drains it. Skin abscesses often start from infected cuts, splinters, or ingrown hairs.

An abscess often won't heal on its own because the pus usually needs to drain out, sometimes with a doctor's help. It's your body doing its job protecting you, but it's also a clear signal that you may need medical attention to finish the fight against infection.