An abscess is an infected cystic lesion formed by infection. The defining feature of an abscess is a collection of purulent fluid (pus) at the location of the infection. The most common types of abscesses are formed in the skin, and include boils, furuncles, and perirectal abscesses among others. Abscesses are generally characterized by swelling, pain, erythema (redness), and often systemic symptoms such as chills or fever.
Abscesses can form in any body structure including the brain, bones, internal organs, muscle tissue, etcetera. They are usually caused by bacteria or other microorganisms specific to the body area. Skin abscesses for example are caused almost exclusively by Staphylococcus or Streptococcus species that colonize the skin. Intra-abdominal abscesses are usually caused by organisms that live in the large intestine. The lungs can host both bacterial and fungal abcesses. More serious abscesses usually require both drainage and treatment with antibiotics to effect a cure. Superficial abscesses often drain spontaneously.