Cellulitis

Cellulitis is a bacterial infection of the dermis, the living layer beneath the skin surface, and the fat below it. It is deeper than impetigo, which stays on the surface, and that depth is why it causes swelling, fever and a genuinely unwell feeling rather than just crusted sores.

Most cases are caused by streptococci, particularly Streptococcus pyogenes and related types, with Staphylococcus aureus responsible for a smaller share and more likely when there is a wound, a boil or a collection of pus involved. The bacteria enter through a break in the skin, then spread sideways through the deeper tissue. There is no clear border to the infection because it is diffusing outward, which is why the edge is often vague rather than sharply defined.

The affected area becomes swollen, warm, tender and discolored, and it expands over hours to days. The skin may look tight, shiny or dimpled like orange peel, and blisters can form on the surface if the swelling is severe. Many people also have fever, chills, a fast heart rate and a general sense of being ill, and these can start before the skin changes are obvious.

Cellulitis is almost always on one side. Around eight in ten cases involve a lower leg, and one leg alone is the pattern. It is not contagious - you cannot catch cellulitis from someone who has it.

A closely related condition, erysipelas, is a more superficial version affecting the upper dermis. It has a sharply raised, clearly defined edge, comes on faster, is more often on the face, and is usually streptococcal. The two overlap and are treated similarly.

This is a section that changes management. Cellulitis is defined in most descriptions by redness, and redness is exactly the sign that is hardest to see on deeper skin tones. Inflammation there often looks brown, violet, gray or simply darker than the surrounding skin, and on very deep skin tones the color change can be subtle enough to be missed entirely, including by clinicians. Cellulitis is diagnosed later on average in people with deeper skin tones as a result, and later diagnosis means a higher chance of ending up in hospital.

The signs to rely on instead do not depend on color. Warmth, felt with the back of the hand and compared with the same spot on the other limb. Swelling, again compared side to side. Tenderness. Tightness or shine to the skin. And spread - an area that is bigger tonight than it was this morning. If you can, mark the edge with a pen and write the time next to it. It is worth saying this directly at the appointment: "this area is warmer, more swollen and more tender than the other leg, and it has grown since this morning." That description carries more weight than asking whether something looks red. Afterward, the skin where the infection was is often darker than the surrounding skin for weeks to months. This is post-inflammatory hyperpigmentation - flat discoloration left behind by inflammation, not remaining infection. It does not need more antibiotics. Peeling and mild scaling as it settles is also normal.

There are three parts to it. First, a break in the skin. It is often small and often overlooked: cracked, scaling skin between the toes from athlete's foot is the classic one, along with cuts, grazes, insect bites, leg ulcers, cracked heels, eczema, surgical wounds and injection sites. Sometimes no entry point is ever found.

Second, bacteria get through that break into the deeper tissue. Streptococci are the usual cause, Staphylococcus aureus less often and more typically when there is a wound or an abscess. Both live on normal skin, so the bacteria are usually already there rather than caught from someone.

Third, the tissue has to let them spread. This is where the risk factors matter more than the exposure. Swollen limbs, poor lymph drainage and poor vein flow all mean fluid sits in the tissue with reduced immune access, so bacteria that would normally be dealt with quietly are able to multiply and move outward. That is why cellulitis so often affects a leg that already swells, and why an episode makes the next one more likely - the infection itself damages lymph channels, which increases swelling, which increases risk again.

The swelling and heat are largely the immune response rather than the bacteria themselves. That is also why the area can look worse for a day or two after treatment starts: as bacteria are killed, they release material that briefly intensifies the reaction.

Cellulitis is mostly about the state of the limb and the barrier, not about hygiene or contact with other people.

Athlete's foot and cracked skin between the toes: The most commonly identified entry point for leg cellulitis, and one of the few genuinely fixable ones.

Cellulitis is one of the faster-moving skin conditions. It advances over hours rather than weeks, and it needs treatment rather than time.

Hours 0 to 24 | Starting: A small area becomes tender, warm and swollen, usually on one lower leg. Discoloration follows. Fever, chills, shivering or simply feeling unwell can come before anything obvious appears on the skin.

Days 1 to 3 untreated | Spreading: The area expands outward with an unclear edge. The skin tightens and may look shiny or dimpled. Blisters can appear. Fever and feeling ill increase.

First 24 to 48 hours of antibiotics | Turning: Fever settles and you begin to feel better before the skin looks better. The skin often looks the same or slightly worse for the first day or two, which is expected. What should stop is the advance of the edge.

Days 3 to 7 of treatment | Settling: Swelling reduces, discoloration begins to fade, tenderness eases. A typical course of oral antibiotics runs about 5 to 7 days, longer if things are slow to improve.

Weeks 1 to 3 | Clearing: The skin peels and flakes as it recovers. Some swelling and discoloration usually remain for a while after the infection has gone.

Weeks to months | Afterward: A darker or lighter mark where the infection was is common and fades slowly, especially on deeper skin tones. Mild lasting swelling in the limb is also common after an episode.

Recurrence is a real feature of this condition - a significant number of people have another episode, particularly if the limb still swells or the athlete's foot was never treated. For people with repeated episodes, doctors sometimes prescribe a long-term low-dose antibiotic to prevent them.

• "Cellulitis is contagious." It is not It is an infection in your own deeper tissue caused by bacteria that mostly already live on your skin. You cannot catch it from someone who has it, and you do not need to keep away from family or work for that reason.
• "Cellulitis and cellulite are related." They are entirely different and only sound similar. Cellulite is the dimpled appearance of normal fat under the skin, most often on thighs and hips. Cellulitis is a bacterial infection that needs antibiotics.
• "Red streaks running up the limb mean blood poisoning." Red or dark streaks tracking upward usually mean the infection has entered lymph channels, which is called lymphangitis. It is not the same as sepsis, but it does mean the infection is progressing and it should be seen the same day rather than the next.
• "If both legs look like this, it must be cellulitis in both." Cellulitis in both legs at the same time is uncommon. Symmetrical discoloration, scaling and itch, usually without fever, is far more often stasis dermatitis from long-term swelling. This is one of the most frequently mistaken diagnoses in skin medicine, and it leads to repeated antibiotic courses that were never going to help.
• "It should look better within a day of antibiotics." The skin often looks the same or worse for the first day or two while you start feeling better in yourself. What should stop is the spreading edge. Fever that persists past 48 hours, or an edge that keeps advancing, is the sign to be reassessed.
• "An antibiotic cream will handle it." It will not. Cellulitis is below the surface, deeper than any cream reaches. It requires antibiotics taken by mouth or given into a vein.
• "Once it has cleared, that is the end of it." Recurrence is common, particularly when the limb still swells or the athlete's foot was never treated. The work that prevents the next episode happens after this one has cleared.

And what is your skin like the rest of the time?

Pick the tile above that looks most like yours and this list narrows to what suits it. The basics stay put - a gentle cleanser, a moisturizer and a daily sunscreen are right whichever kind you have.

Cellulitis always needs a doctor. There is no over-the-counter version of this treatment and no benefit to waiting - the infection spreads over hours, and early antibiotics are what keep it from becoming a hospital admission.

**Go to an emergency department now if any of the following are present.** Pain that is far more severe than the skin appearance seems to justify. Skin turning dusky, purple, gray or black, or areas that go numb. Blisters filled with dark fluid, or skin that is breaking down. A crackling feeling under the skin when it is pressed. Redness or discoloration spreading visibly while you watch, over minutes to hours. These can indicate necrotizing fasciitis, a rare but life-threatening deep infection that needs surgery, not just antibiotics.

**Also go to an emergency department the same hour** if there are signs the infection has gone systemic: a high fever with shaking chills, confusion or new drowsiness, a very fast heart rate, breathing quickly, feeling faint or unable to stand, passing little or no urine, or feeling dramatically and rapidly worse. Cellulitis can lead to sepsis, and sepsis is time-critical.

**Get urgent same-day care** for cellulitis around the eye or on the face, where the infection can spread behind the eye and threaten sight - particularly with a swollen eyelid, pain on moving the eye, double vision or reduced vision. Also seek same-day care for cellulitis in a baby or young child, in anyone with a weakened immune system, in anyone with diabetes or poor circulation, after an animal or human bite, after exposure to sea water, fresh water or fish handling, or when dark streaks are tracking up the limb.

**Book a same-day appointment** for the ordinary presentation: an area of skin on one limb that is swollen, warm, tender and discolored, and larger than it was earlier in the day, with or without fever.

**Go back within 48 hours** if you have started antibiotics and the fever has not settled, the edge is still advancing, or you feel worse rather than better. Not improving usually means the wrong antibiotic, a collection of pus that needs draining, or a different diagnosis - and all three need reassessing rather than waiting out.

Cellulitis is diagnosed clinically. There is no single test that confirms it, so the diagnosis rests on what the doctor sees and feels: an area of skin on one limb that is swollen, warm, tender and discolored, with an indistinct edge, that has been expanding, often with fever or feeling unwell.

The examination has a few specific parts. The doctor compares the two limbs side by side, feels for warmth and swelling, looks for an entry point - between the toes, on the heel, an ulcer, a bite, a wound - and checks for tender lymph nodes in the groin or armpit and for streaks tracking up the limb. The border is often marked with a pen so that progress can be measured objectively over the following day.

Blood tests are used when someone is unwell, to look at inflammatory markers, kidney function and whether the infection has spread into the bloodstream. Blood cultures are positive in only a small minority of cases and are usually reserved for people who are systemically ill, immunosuppressed, or have had unusual exposures. Swabbing intact skin is not useful, because it grows the bacteria that live there normally. A swab of an open wound, ulcer or drained pus can be genuinely helpful. If an abscess is suspected under the skin, an ultrasound scan finds collections that are not obvious on examination, and pus that is found needs draining as well as antibiotics.

The most important part of the assessment is what else it could be. A meaningful proportion of people diagnosed with cellulitis in fact have something else, and both legs being affected is the biggest clue. Stasis dermatitis from long-term leg swelling is by far the most common impostor - it is usually bilateral, itchy rather than painful, scaly, and comes without fever. A deep vein thrombosis causes a swollen, tender, warm calf and can look very similar, which is why a scan is sometimes needed. Contact dermatitis follows the shape of whatever touched the skin and itches. Gout, an inflamed joint, lipodermatosclerosis, an inflammatory reaction to an insect bite and a superficial clot in a vein all get mistaken for it. Telling a doctor how quickly it came on, whether it itches or hurts, and whether you have a fever helps separate these more than anything else.

1. Lee RA, Centor RM, Humphrey LL, et al. Appropriate Use of Short-Course Antibiotics in Common Infections: Best Practice Advice From the American College of Physicians. Annals of internal medicine. 2021. — Annals of internal medicine, 2021
2. Dalal A, Eskin-Schwartz M, Mimouni D, et al. Interventions for the prevention of recurrent erysipelas and cellulitis. The Cochrane database of systematic reviews. 2017. — The Cochrane database of systematic reviews, 2017
3. Cutler TS, Jannat-Khah DP, Kam B, et al. Prevalence of misdiagnosis of cellulitis: A systematic review and meta-analysis. Journal of hospital medicine. 2023. — Journal of hospital medicine, 2023
4. Nightingale R, Yadav K, Hamill L, et al. Misdiagnosis of Uncomplicated Cellulitis: a Systematic Review and Meta-analysis. Journal of general internal medicine. 2023. — Journal of general internal medicine, 2023
5. Taira KG, Wang M, Guo W, et al. Association of Cellulitis With Obesity: Systematic Review and Meta-Analysis. JMIR dermatology. 2024. — JMIR dermatology, 2024