Boils can occur anywhere on your skin, but appear mainly on your face, neck, armpits, buttocks or thighs — hair-bearing areas where you're most likely to sweat or experience friction. Signs and symptoms of a boil usually include:
- A painful, red bump that starts out about the size of a pea
- Red, swollen skin around the bump
- An increase in the size of the bump over a few days as it fills with pus (can sometimes reach the size of a baseball)
- Development of a yellow-white tip that eventually ruptures and allows the pus to drain out
A carbuncle is a cluster of boils that form a connected area of infection. Carbuncles often occur on the back of the neck, shoulders or thighs. Compared with single boils, carbuncles cause a deeper and more severe infection and are more likely to leave a scar. People who have a carbuncle often feel unwell in general and may experience a fever and chills.
When to see a doctor
You usually can care for a single, small boil yourself. But see your doctor if you have more than one boil at a time or if a boil:
- Occurs on your face
- Worsens rapidly or is extremely painful
- Causes a fever
- Is more than 2 inches (5 centimeters) across
- Hasn't healed in two weeks
Most boils are caused by Staphylococcus aureus, a type of bacteria commonly found on the skin and inside the nose. Boils sometimes develop at sites where the skin has been broken by a small injury or an insect bite, which gives the bacteria easy entry.
Although anyone — including otherwise healthy people — can develop boils or carbuncles, the following factors can increase your risk:
- Close contact with a person who has a staph infection. You're more likely to develop an infection if you live with someone who has a boil or carbuncle.
- Diabetes. This disease can make it more difficult for your body to fight infection, including bacterial infections of your skin.
- Other skin conditions. Because they damage your skin's protective barrier, skin problems, such as acne and eczema, make you more susceptible to boils and carbuncles.
- Compromised immunity. If your immune system is weakened for any reason, you're more susceptible to boils and carbuncles.
Rarely, bacteria from a boil or carbuncle can enter your bloodstream and travel to other parts of your body. The spreading infection, commonly known as blood poisoning (sepsis), can lead to infections deep within your body, such as your heart (endocarditis) and bone (osteomyelitis).