Bacterial skin infections and how to spot them early
How to tell a bacterial skin infection from ordinary irritation, what cellulitis, impetigo and boils look like, and the red flags that need urgent care.
A bite on your ankle that you scratched in your sleep is now a warm red patch the size of a plum. On Monday it itched. By Wednesday it is sore rather than itchy, the skin feels tight and shiny, and pressing it makes you wince. That change, from itchy to sore, is one of the most useful signals your body gives you.
Most red skin is not infected. Irritation, eczema and allergic reactions all look inflamed, and all of them settle on their own. A bacterial infection is a different process, because bacteria have got through the skin barrier and are multiplying in the tissue underneath, and that one carries on until it is treated. Working out which of the two you are looking at matters more than knowing its name.
The four you are most likely to meet
Cellulitis
An infection of the deeper skin and the tissue just beneath it, usually on a lower leg, sometimes an arm or the face. The area is red, hot to the touch, swollen and tender, and the redness has no crisp border, it just fades into normal skin. It often follows something trivial: a crack between the toes, a scratch, an insect bite. People frequently feel shivery and unwell before the skin looks dramatic.
Impetigo
Very superficial, very contagious, and more common in children although certainly not limited to them. Small blisters or sores break and dry into a golden brown crust that looks almost stuck on, often around the nose and mouth. It travels on fingers and towels, which is why it turns up in more than one member of a household.
Infected cuts and bites
A wound that was improving and then reverses is the classic story. Pain increasing after the second day, redness creeping out from the edges, yellow discharge, a smell, or wound edges pulling apart. Animal and human bites deserve more respect than their size suggests, particularly on a hand, because the puncture seals over and traps bacteria deep in a place full of tendons and joints.
Boils and abscesses
A painful lump that swells over a few days and comes to a head, usually starting in a hair follicle. Small ones often settle with warm compresses and time. A large one, or one that is exquisitely painful, may need draining, which is not a job to attempt yourself with a needle at the bathroom mirror. Our page on how skin infections are assessed and treated covers each of these in more detail.
Irritation or infection
The distinction is usually about direction of travel rather than how things look on any single day.
| Feature | Ordinary irritation | Likely infection |
|---|---|---|
| Main sensation | Itching | Pain, and increasing |
| Temperature | Same as nearby skin | Noticeably hot |
| Over two days | Settles or stays put | Spreads outwards |
| The edge | Often matches what touched you | Vague and expanding |
| How you feel | Well in yourself | Shivery, tired, off your food |
| Both sides of the body | Common | Rare, usually one area |
If it itches more than it hurts, is not hot, and you feel perfectly well in yourself, you are probably looking at a reaction rather than an infection, and our guide to rashes and allergic skin reactions will be the more useful read.
Draw round the edge. Genuinely.
This is the most useful thing in this article. Take a pen, draw a line on the skin around the outer limit of the redness, and write the time next to it. Six hours later you know something you could never have learned by staring at it: whether the infection is advancing, holding steady, or retreating. Photograph it each time, because the ink smudges.
Redness that has crossed your line in a few hours is the finding that changes what happens next, and it is worth far more to a doctor than any adjective you could offer. Redness sitting well inside the line a day after treatment started is reassurance you can trust. Memory is a poor measuring device for something moving this slowly, and without a mark to compare against most people either panic or reassure themselves, usually wrongly.
When to get urgent help
- Redness spreading quickly, particularly if it has moved noticeably in a few hours.
- Fever, shivering or shaking chills alongside the skin problem.
- Red streaks running up a limb away from the area, or tender lumps in the groin or armpit.
- Pain that is severe and out of all proportion to how the skin looks, especially in someone who is very unwell. This one needs immediate attention.
- Any infection around the eye, or spreading across the face.
- Infection in anyone with diabetes, a weakened immune system or on medicines that suppress immunity, and any wound over a joint or on a hand.
- Skin turning dusky, purple or black, or blistering across the red area.
Call 999 or go to an emergency department for pain that is severe and out of proportion to the appearance, for skin turning dusky or black, or for someone who is confused, very unwell or struggling to stay awake. Use 111 when you need advice quickly and it is not an emergency, for instance redness creeping gradually with a mild temperature.
How an online consultation works for this
Infections are time sensitive, which is the main argument for not leaving it until Monday. In a video appointment a doctor looks at the area, asks how it started and what it has done since, whether you have a temperature and how you feel in yourself, and takes your other conditions and medicines into account. Where a bacterial infection is the likely cause and treatment is clinically appropriate, they can prescribe. Where it is not bacterial they will say so, because an antibiotic does nothing whatsoever for a viral rash or an irritated patch of eczema. If your skin needs hands on care, drainage or a hospital assessment, you will be told clearly and given a referral letter where that is the right route. The steps involved in a video appointment take a minute to read, fees are on our consultation pricing page, and if the redness is on the move you can get seen today by a UK registered doctor.
Have the useful things ready before you join: photographs from the last day or two, your inked outline, your temperature if you have taken it, and a list of your regular medicines. It takes two minutes and makes the appointment considerably more useful.
Keeping small things small
- Wash any break in the skin under clean running water as soon as it happens, then cover it.
- Change a dressing when it is wet or dirty rather than on a schedule.
- Treat athlete's foot properly, because cracks between the toes are a common doorway for cellulitis further up the leg.
- Do not share towels, razors or flannels while anyone in the house has crusted sores.
- Keep moisturising eczema even when it is calm, since intact skin is the barrier that keeps bacteria out.
- If you have diabetes, look at your feet properly once a day, including between the toes.
You can also see the wider range of conditions our doctors handle, and if the swelling and pain are really in your jaw rather than your skin, help with tooth pain and dental infection is the better fit. Infections do not wait for a convenient moment, though, and a day of hesitating is sometimes the difference between a straightforward course of treatment and a hospital trip. If your skin is hot, spreading or hurting more with every hour, speak to a doctor today.