A boil (furuncle) is a pus-filled bump in the skin that is caused by a bacterial infection. It’s a bit like a very big yellow pimple, but it’s deeper in the skin and hurts a lot more.

Boils develop when a hair follicle and the surrounding tissue become infected. Hair follicles consist of one hair, the root of the hair, a sebaceous gland and a small muscle that can pull the hair up, making it stand on end. Hair follicle inflammations are sometimes also referred to as “deep folliculitis” or “perifolliculitis.”

The infection causes the skin tissue inside the boil to die, creating a pus-filled hollow space (an abscess). Skin abscesses can develop from boils, but also from other things like infected insect bites or injections with dirty needles. If several boils merge into a larger bump, it’s called a carbuncle.

Sometimes boils go away again on their own, without causing any problems. But it’s often a good idea to get medical treatment. This can help make boils go away quicker, relieve the pain and prevent complications.

Boils are caused by bacteria, most commonly by Staphylococcus aureus bacteria (a staph infection). A lot of people have these bacteria on their skin or – for instance – in the lining of their nostrils, without them causing any problems.

They are more likely to lead to boils or other skin infections in people who have weakened immune systems. For this reason, boils are more common in people with medical conditions such as diabetes, chronic infections or cancer. They are also more common in people with eczema, conjunctivitis or certain allergies such as allergic asthma.

Boils are painful swollen bumps, ranging from roughly the size of a cherry stone to that of a walnut. They feel warm and look red, and yellowish pus may show through the skin. If a cluster of boils (a carbuncle) develops, the infection might cause a fever too, making you feel weak and tired.

Boils mainly occur on the face and neck, including the back of the neck. But they sometimes also develop in the armpits, groin, genital area, on the back, bottom or thighs.

Doctors usually recognize boils based on their typical appearance and a description of the symptoms. Further diagnostic procedures such as blood tests or a pus swab are only needed if someone often gets boils, has several boils at the same time, or is thought to be at high risk of complications.

The pus is examined in a laboratory in order to find out exactly what kind of bacteria are causing the infection, and determine which antibiotics are most likely to work the best. Blood tests help to find out whether the infection has already spread and whether the person has any other medical conditions that could increase the risk of bacterial infections occurring.

Boils are usually treated by a doctor. The treatment typically involves opening the pus-filled abscess with a small cut, draining the pus, disinfecting the wound and placing strips of sterile gauze inside it to soak up and remove any remaining pus. The wound stays open while healing, so it doesn’t have to be sewn shut. It’s very important not to squeeze boils yourself – especially if they’re on your face.

If a boil is still growing in size, the abscess hasn’t yet fully developed. Doctors can feel the boil with their hands to see whether that is the case. In this phase, you can try to start or speed up the healing process by applying a warm, moist cloth or a special ointment that draws (pulls) pus out of the boil. This kind of ointment is also known as "drawing salve".

Antibiotics are only needed if complications are likely or have already occurred – for instance, if several boils have merged and developed into a carbuncle. People then sometimes go to the hospital to get antibiotics through a drip (an infusion). It is particularly important that carbuncles are cut open so that the pus can escape. The wound is then repeatedly cleansed using an antiseptic solution.

If you squeeze a boil or scratch it open, the bacteria might spread in the body along the blood or lymph vessels. If, for instance, you can see a red streak leading away from the boil, it means that the infection is moving along a lymph vessel (lymphangitis). Lymph nodes in the affected area may also become inflamed and hurt (lymphadenitis).

People sometimes think that the red streak caused by lymphangitis is a sign of blood poisoning (sepsis). But this very rare, severe complication only develops if a lot of bacteria enter the blood at once and quickly spread throughout the whole body.

If boils occur on the face – particularly around the nose and upper lip area – there’s a certain risk that the bacteria might get into the brain, where they could lead to meningitis or life-threatening blood clots in the large blood vessels (called cerebral venous sinus thrombosis, or CVST).

Especially in people with a weakened immune system, boils can keep coming back or occur in several different places at the same time. Doctors call this furunculosis.

If several boils develop in neighboring hair follicles and merge into a larger connected area of infection under the skin, it’s called a carbuncle. Carbuncles often occur at the back of the neck, and go deeper into the tissue than boils do.