A boil is a red, painful, lump on the skin that develops at the site of an infected hair follicle. A hair follicle is a small sac in the skin that a hair grows out of.
As white blood cells fight the infection, pus forms inside the boil and the boil grows larger and becomes more painful. Eventually, it will burst and the pus will drain away. This can take from two days to three weeks to happen.
A carbuncle is a collection of boils that develop in a group of hair follicles under the skin.
Read more about the symptoms of boils and carbuncles.
Causes of boils and carbuncles
Boils and carbuncles are caused by a type of bacteria known as staphylococcus aureus (staph bacteria). Staph bacteria usually live harmlessly on the surface of the skin or in the lining of the nose. However, if they get inside the skin, they can trigger skin infections, such as boils.
Read more about the causes of boils and carbuncles.
Who is affected by boils and carbuncles?
Boils are relatively common in teenagers and young adults, usually in males. Young males living in over-crowded and possibly unhygienic conditions, such as a military base or prison, are particularly at risk (see box, left).
Carbuncles are less common and tend to occur mostly in middle-aged or older men who are in poor health due to a pre-existing health condition, such as heart disease.
Read more about the causes and risk factors for boils and carbuncles.
When to see your GP
Most boils burst and heal by themselves without the need for medical treatment. However, you should visit your GP if you have a boil:
- on your face or nose
- that gets bigger and feels soft and spongy to touch (it may not burst and heal by itself)
- that doesn't heal within two weeks
You should be able to successfully treat a boil at home. One of the best ways to speed up healing is to apply a warm facecloth to the boil, three or four times a day.
If your boil doesn't heal, your GP may decide to drain it.
Never attempt to squeeze or pierce a boil or carbuncle because it could cause the infection to spread and may lead to complications (see below).
Carbuncles are usually treated with a course of antibiotics.
Boils and carbuncles can cause secondary infections.
This can range from a relatively minor (though often very painful) infection of the deeper layer of the skin, such as cellulitis, to rarer more serious infections, such as blood poisoning (sepsis).
Boils around the nose or eyes can spread to the brain and cause serious injury.
Larger boils and carbuncles can also lead to scarring, particularly if they're not treated by a properly qualified healthcare professional.
Read more about the complications of boils and carbuncles.