Most abscesses are caused by a bacterial infection. However, they can very occasionally develop due to an infection by viruses, fungi or parasites.

When foreign organisms such as bacteria enter the body, the immune system sends white blood cells to fight the infection. This causes swelling (inflammation) at the site of infection and the death of nearby tissue, creating a hole called a cavity, which fills with pus to form an abscess.

The pus contains a mixture of dead tissue, white blood cells and bacteria. The abscess may get larger and more painful as the infection continues and more pus is produced.

Most abscesses occur as a result of staphylococcal bacteria and streptococcal bacteria.

Some types of staphylococcal bacteria produce a toxin called Panton-Valentine leukocidin (PVL), which kills white blood cells. This causes the body to make more cells to keep fighting the infection, and can lead to repeated skin infections.

Read about the symptoms of staphylococcal infections.

Skin abscesses

When bacteria gets under the surface of your skin, an abscess can form. This can occur anywhere on the body, although skin abscesses tend to be more common in the:

• underarms
• hands and feet
• trunk
• genitals
• buttocks

Bacteria can get into your skin and cause an abscess if you have a minor skin wound, such as a small cut or graze, or if a sebaceous gland (oil gland) or sweat gland in your skin becomes blocked.

Internal abscesses

Abscesses that develop inside the tummy (abdomen) are caused by an infection reaching tissue deeper within the body. This can occur as a result of:

• an injury
• abdominal surgery
• an infection spreading from a nearby area

There are many ways an infection can spread into the abdomen and cause an abscess to develop.

For example, a lung abscess can occur due to a bacterial infection in your lungs, such as pneumonia, and a burst appendix can spread bacteria within your abdomen.

Increased risk

In addition to the specific causes mentioned above, things that increase the likelihood of an abscess developing can include:

• having a weakened immune system – this could be due to a medical condition such as HIV, or a treatment such as chemotherapy
• having diabetes
• having an underlying inflammatory condition, such as hidradenitis suppurativa, panniculitis, acute pancreatitis and inflammatory bowel disease (IBD)
• being a carrier of staphylococcal bacteria

However, many abscesses develop in people who are otherwise generally healthy.