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Look for a red, painful bump in your skin. Note whether the bump grows larger in the days after it appears. Check for yellowish pus under the skin in the middle of the bump. Watch out for more severe symptoms that may indicate a carbuncle.
When a boil first begins to develop, the infection will remain fairly deep under the skin. Boils usually start out as a reddish bump about the size of a pea that is painful to the touch. In some cases, the boil may hurt even when you are not touching it.  The skin around the bump may appear swollen and inflamed. Boils can appear anywhere on your body, but they are most likely to develop in areas where you experience a lot of sweating and friction. Common places include the face, neck, armpits, thighs, and buttocks. Keep an eye on the possible boil in the next few days after you first notice it. If it is a boil, it will begin to expand as the abscess under your skin fills with pus. Some boils can grow to around the size of a baseball, but this is uncommon.  You can monitor the growth of the boil by putting a pen mark at its edge so you can see if it expands. Alternatively, you can measure it every day. As the boil grows, it will typically become more painful and softer to the touch. As the boil grows, look for the formation of a yellow or off-white “tip.” This can happen when the pus inside the boil comes to the surface and becomes visible under your skin. In many cases, the pustule will rupture on its own, allowing the boil to drain and heal.  Keep in mind that you may not see the pus if the boil is fresh. Usually, the pus does not become visible until the later stages of the boil. Never attempt to pierce or squeeze the boil to drain the pus. Doing so could cause the infection to spread deeper into your tissues. If you notice that you have what appears to be several boils clustered together, you may have a carbuncle. These infections most commonly appear on the shoulders, back of the neck, or thighs. In addition to pain and swelling, look out for symptoms such as fever, chills, and a general feeling of illness.  A carbuncle may be as big as 4 inches (10 cm) across. They typically take the form of a large, swollen area with a dense cluster of pustules at the highest point.  Carbuncles or severe boils may also cause swelling in your nearby lymph nodes.