An infected boil looks like a painful, red or purplish bump with swollen skin around it, often developing a yellow-white tip as pus collects inside. It starts small and can grow to more than 2 inches (5 centimeters) across over several days. If you’re staring at a tender lump on your skin and wondering what it is, here’s what to look for at each stage and when the appearance signals something more serious.
Early Appearance
A boil begins as a firm, red, tender bump, usually about the size of a pea. The skin around it feels warm and looks slightly swollen. At this stage it can easily be mistaken for a large pimple or an insect bite. The key difference is depth: a boil sits deeper in the skin than a typical pimple and feels like a hard knot under the surface rather than a superficial whitehead.
Over the first day or two, the redness spreads outward from the center of the bump, and the surrounding skin takes on a puffy, reddish or purplish tone. The area becomes increasingly painful, especially when touched or when clothing presses against it.
How It Changes Over Several Days
As the infection progresses, the bump fills with pus and grows noticeably larger. Most boils reach their full size within three to seven days. During this time you’ll see the lump become softer and more dome-shaped as the pressure inside builds. The skin stretching over the top may look shiny or taut.
The hallmark of a mature boil is the development of a yellow-white tip, sometimes called a “head” or “point.” This is the thinnest part of the skin barrier between the pus pocket and the surface. Eventually, this tip ruptures on its own and thick, yellowish or greenish pus drains out, often mixed with a small amount of blood. Once the pus drains, pain drops quickly and the area starts to flatten and heal.
Where Boils Typically Show Up
Boils form in hair follicles, so they appear in areas where friction, moisture, and hair combine. The most common spots are the face, neck, armpits, inner thighs, and buttocks. You can also get them on the groin or anywhere clothing rubs repeatedly against the skin. If a bump appears in one of these friction-prone zones and matches the progression described above, a boil is the most likely explanation.
Single Boil vs. Carbuncle
A single boil (sometimes called a furuncle) is one infected hair follicle forming one pocket of pus. A carbuncle is a cluster of boils connected beneath the skin, creating a larger, deeper area of infection with multiple drainage points on the surface. Carbuncles look angrier: the swelling covers a wider area, the skin may appear deeply red or purple, and you might notice pus oozing from several openings rather than one central tip. Carbuncles are more likely to cause fever, fatigue, and general malaise, and they typically require professional drainage.
Can You Tell if It’s MRSA?
You can’t. Both regular staph infections and drug-resistant staph (MRSA) create boils that look identical: red, swollen, painful, warm, and full of pus. There is no visual feature, color difference, or size threshold that distinguishes one from the other. The only way to identify MRSA is through a lab test on a sample of the pus. This matters because MRSA doesn’t respond to the most commonly prescribed antibiotics, so if a boil isn’t improving or keeps coming back, testing helps guide the right treatment.
Signs the Infection Is Spreading
A simple boil stays localized: the redness and swelling stay close to the bump. If the infection starts moving into the surrounding tissue, the appearance changes in specific ways that are worth watching for.
- Expanding redness. The red, warm zone around the boil grows noticeably larger over hours rather than days, sometimes with an irregular border that creeps outward. This pattern suggests cellulitis, a skin infection that can spread rapidly.
- Red streaks. Lines of redness radiating away from the boil toward the nearest lymph nodes (for example, from your thigh toward your groin) indicate the infection is tracking along lymphatic channels.
- Skin changes. Blisters, dimpling, or a pitting texture in the surrounding skin are signs of deeper tissue involvement.
- Fever and chills. A boil that’s just a local pocket of pus rarely causes a fever. If you develop a temperature above 100.4°F (38°C), chills, or feel generally unwell, the bacteria may be entering the bloodstream.
- Rapid swelling. A rash or swelling that changes visibly within hours rather than days needs urgent attention.
What Happens During Treatment
Small boils often resolve on their own. Applying a warm, moist compress for 20 minutes several times a day increases blood flow to the area and helps the boil come to a head and drain naturally. Once it opens, keeping it clean and covered with a bandage is usually enough.
Larger boils, or those that haven’t drained after a week or two of home care, may need to be opened by a healthcare provider. The procedure is straightforward: the area is numbed, a small cut is made, and the pus is drained. Packing material is sometimes placed inside the cavity to keep it open and allow any remaining infection to drain over the next day or two. Relief is usually immediate once the pressure is released.
Antibiotics alone don’t cure most boils. Draining the pus is the primary treatment. Antibiotics are added when there are signs of spreading infection, a fever, multiple boils, a weakened immune system, or when the boil hasn’t responded to drainage alone. If MRSA is suspected or confirmed through lab testing, specific antibiotics that target resistant bacteria are prescribed.
Boil vs. Other Skin Bumps
A cyst feels like a smooth, round lump under the skin and is usually painless unless it becomes infected. It doesn’t develop the angry redness or yellow tip of a boil. An ingrown hair creates a smaller, shallower bump, often with a visible hair trapped beneath the surface. A large pimple sits near the skin’s surface and comes to a head faster, while a boil builds deeper and takes longer to mature. Hidradenitis suppurativa causes recurring boil-like lumps in the armpits and groin, but these tend to leave tunnels or scars under the skin and come back in the same areas repeatedly over months or years.
The combination of location (a hair-bearing, friction-prone area), depth (a firm lump that sits well below the surface), progressive growth over days, and eventual formation of a pus-filled tip is what sets an infected boil apart from other skin conditions.