Impetigo and Cellulitis: How to Tell Them Apart and Choose the Right Antibiotic

Two kids come home from school with red, crusty sores around their noses. A neighbor wakes up with a swollen, hot patch on her leg that feels like it’s burning. Both have skin infections-but they’re not the same. One is impetigo, the other is cellulitis. They look similar at first glance, but the way they spread, how deep they go, and which antibiotics work best are completely different. Get it wrong, and you risk turning a simple rash into a life-threatening infection.

What Impetigo Really Looks Like

Impetigo is the most common bacterial skin infection in children, especially between ages 2 and 5. You’ll see it in daycare centers, schools, and households where kids share towels or toys. It starts as tiny red bumps or blisters, usually around the nose and mouth, but can show up anywhere the skin is broken-from a scratch to an eczema flare-up.

The classic sign? Honey-colored crusts. These aren’t just dried pus-they’re thick, sticky, and golden-yellow, formed when fluid from ruptured blisters dries on the skin. This is called nonbullous impetigo, and it makes up about 70% of cases. The rest are bullous impetigo, which features larger, fluid-filled blisters that burst easily, leaving a ring-like edge. Both are caused by Staphylococcus aureus or Streptococcus pyogenes.

What makes impetigo tricky is how fast it spreads. One child can infect half a classroom in a week. It doesn’t need broken skin to start-S. aureus can invade healthy skin, especially in warm, humid weather. That’s why it spikes in summer and early fall. Parents often mistake it for a spider bite or allergic reaction. But if you see crusts forming over a day or two, and it’s spreading, it’s likely impetigo.

Cellulitis: When the Infection Goes Deep

Cellulitis doesn’t just sit on the surface. It burrows into the dermis and fat layer under the skin. It’s not a rash. It’s an invasion. You’ll know it by the way the skin feels: hot, swollen, tight, and tender to the touch. The redness doesn’t have clean edges like a ringworm-it bleeds outward in a messy, spreading pattern. Sometimes, the area leaks clear or yellow fluid. Fever, chills, or feeling generally sick? That’s a red flag.

It most often hits the lower legs, but can appear anywhere-arms, face, even around surgical wounds. People with diabetes, poor circulation, or eczema are at higher risk. A small cut, insect bite, or even a cracked toe can be the entry point. Unlike impetigo, cellulitis rarely spreads person-to-person. It’s an individual breakdown of the body’s defenses.

There’s a related condition called erysipelas, which looks like cellulitis but has sharp, raised borders and is almost always caused by strep. It’s more common on the face and feels like a sunburn with a ridge around it. Doctors treat it like cellulitis, but the sharper edge helps tell them apart.

Why Antibiotic Choice Matters

Both infections are bacterial, but they need different weapons. Give someone impetigo the wrong antibiotic, and it might clear up slowly-or come back. Give someone cellulitis a weak one, and it could turn into sepsis.

In the UK and Belgium, flucloxacillin is the go-to first-line drug for both. It’s a penicillin-type antibiotic that kills staph and strep-exactly the bugs behind most cases. It’s cheap, effective, and well-tolerated. But if you’ve had antibiotics before, or the infection isn’t improving, resistance becomes a problem. That’s where MRSA (methicillin-resistant Staphylococcus aureus) comes in.

MRSA doesn’t respond to flucloxacillin, amoxicillin, or most common penicillins. In the US and parts of Europe, MRSA now makes up 30-50% of skin infections in some hospitals. If someone has a history of hospital stays, recent surgery, or lives in a crowded setting like a prison or nursing home, doctors suspect MRSA early.

For MRSA, you need alternatives: clindamycin, doxycycline, or trimethoprim-sulfamethoxazole. These are broader-spectrum and reserved for resistant cases. In severe cellulitis-with fever, rapid spread, or immune problems-patients get IV antibiotics like vancomycin in the hospital.

France takes a different path. They use amoxicillin for cellulitis and pristinamycin for severe impetigo. Why? Because resistance patterns vary by region. What works in Leeds might not work in Marseille. That’s why local guidelines matter more than national ones.

Topical vs. Oral: When to Use What

For small, isolated patches of impetigo, topical treatment works just as well as pills. Mupirocin ointment (sold as Bactroban) is applied three times a day for 7-10 days. Studies show it clears up 90% of non-bullous cases. No need to swallow antibiotics if you don’t have to.

But if the infection covers a large area, is spreading fast, or involves multiple family members, oral antibiotics are better. Flucloxacillin (or clindamycin if MRSA is suspected) for 7-10 days is standard. Kids often get liquid forms, adults get capsules.

Cellulitis? Never just a cream. Even if the red area looks small, the infection is already under the skin. Oral antibiotics for 5-14 days are required. If the person is vomiting, has a high fever, or can’t walk because their leg is swollen, they need hospital care and IV antibiotics. Delaying treatment by more than 48-72 hours increases the risk of complications like abscesses, blood infections, or tissue death.

What Happens If You Wait Too Long

Impetigo can heal on its own in a few weeks-but it won’t stop spreading. Kids keep scratching, touching their faces, and then their toys. Parents end up infected too. In rare cases, strep-caused impetigo can lead to post-streptococcal glomerulonephritis-a kidney problem that shows up weeks later with swelling, dark urine, and high blood pressure.

Cellulitis is far more dangerous. If untreated, the bacteria can enter the bloodstream, causing sepsis. Or they can spread to deeper tissues, leading to necrotizing fasciitis-the “flesh-eating” infection. Even without that, recurrent cellulitis is common. One in three people who’ve had it once will get it again within two years. That’s why doctors look for underlying causes: foot ulcers in diabetics, leg swelling from poor veins, or chronic skin conditions.

Prevention: Simple Rules, Big Impact

You can’t always stop an infection-but you can stop it from spreading.

• Wash hands often, especially after touching infected skin.
• Cover sores with clean bandages until they crust over.
• Don’t share towels, clothing, or bedding with someone infected.
• Keep cuts and scrapes clean and covered. Use soap and water, then apply an antiseptic.
• If you have eczema, manage it well. Dry, cracked skin is a gateway for bacteria.
• Children with impetigo should stay home from school or daycare until 24 hours after starting antibiotics.

For people with recurrent cellulitis, long-term low-dose antibiotics (like penicillin V) may be prescribed to prevent future episodes. Compression socks help if poor circulation is the cause. Diabetics need regular foot checks.

When to See a Doctor

Don’t wait for it to get worse.

• Impetigo: If sores spread beyond one area, don’t improve after 3 days of treatment, or if your child has a fever.
• Cellulitis: If the red area is growing fast, you have chills or fever, the skin feels numb or looks purple, or you have diabetes or a weakened immune system.

Emergency care is needed if you feel dizzy, have trouble breathing, or the infection is near your eyes or genitals. These areas are high-risk for rapid spread.

What Comes Next

Antibiotic resistance is changing how we treat these infections. More doctors are now taking swabs before prescribing-especially for recurrent or treatment-resistant cases. Rapid tests that identify staph vs. strep and detect MRSA in under an hour are becoming available in hospitals. In the next five years, we’ll likely see fewer broad-spectrum antibiotics and more targeted treatments based on real-time lab results.

For now, the best defense is awareness. Know the difference between a crusty sore and a hot, swollen patch. Treat impetigo early to stop the spread. Treat cellulitis aggressively to stop the danger. And never assume a skin infection is “just a rash.” It might be the first sign of something much bigger.