Most boils and carbuncles develop when the hair follicles in your skin become infected with bacteria.
A hair follicle is a cavity in the skin that a hair grows from.
Boils are usually caused by Staphylococcus aureus bacteria (also known as staph bacteria) infecting one or more hair follicles.
Staph bacteria commonly live harmlessly on the skin and inside the nose and throat. It is estimated that about 20% of otherwise healthy people are long-term carriers of staph bacteria.
Boils tend to occur when the bacteria enters the skin through a cut or graze. This causes your immune system to respond by sending infection-fighting white blood cells to the source of the infection to kill the bacteria.
Over time, a mix of dead bacteria, dead white blood cells and dead skin cells builds up inside the boil to form pus.
Things that make you more likely to get boils include:
- being male (particularly a teenage boy) – this could be because hormonal changes during puberty can make the skin greasy, which may encourage the growth of bacteria
- being in close personal contact with someone who has a boil
- taking part in sports that involve a combination of sweating, close personal contact and frequent friction of the skin, such as rugby and wrestling
- living in conditions that are overcrowded and have poor standards of personal hygiene, or both
- having a pre-existing skin condition, such as atopic eczema or scabies
- obesity – being very overweight with a body mass index (BMI) of 30 or above
However, it is also common for healthy people with good levels of hygiene to develop boils at some point.
Like boils, carbuncles are usually caused by Staphylococcus aureus bacteria. A carbuncle develops when the infection spreads further beneath the skin to create a cluster of boils.
People in poor health or those with a weak immune system (the body's natural defence against infection and illness) are thought most at risk of developing a carbuncle. These include people:
- with diabetes – high sugar levels in the blood make it more difficult for your immune system to protect you against skin infection
- with health conditions known to weaken the immune system, such as HIV
- who are on a long-term course of steroid tablets or injections (corticosteroids) – long-term steroid use makes you more vulnerable to infection
- who are having treatment known to weaken the immune system, such as chemotherapy
- who are malnourished (not getting the nutrients needed from food to maintain good health)
- with skin conditions that affect a large part of their body
- with heart disease
- with a severe drug misuse problem, particularly those who inject drugs