Impetigo starts when bacteria already living on the skin’s surface get pushed through a break in the skin, often from a scratch, insect bite, or minor scrape. The first visible sign is usually a small cluster of reddish sores, most often around the nose and mouth, that appear about 10 days after the initial bacterial exposure. From there, the sores follow a predictable pattern that depends on which type of impetigo develops.
What Happens Before Sores Appear
The bacteria behind impetigo, typically staph or strep, don’t start inside a wound. They colonize normal, unbroken skin first. Detailed studies of strep-caused impetigo have shown that the bacteria settle on the skin surface an average of 10 days before any sores develop. During this invisible colonization phase, the bacteria use surface proteins to latch onto skin cells. One protein acts as a first-step anchor, pulling the bacteria close to skin cells through a kind of molecular stickiness. Once in close contact, other proteins lock onto the outer layer of skin cells for a tighter hold.
The sores themselves most likely form when these surface bacteria get pushed into deeper layers of skin through minor trauma: a bug bite you scratched, a scrape from playing outside, a patch of eczema you’ve been picking at, or even a small cut you barely noticed. Any disruption to the skin barrier can serve as an entry point. This is why impetigo is so common in children, who tend to have more scrapes, bug bites, and habits like nose-picking that create opportunities for bacteria to get beneath the surface.
The Classic First Signs
The most common form, called non-bullous impetigo, starts as one or more tiny blisters or pus-filled bumps. These are small and easy to miss at first. Several of these tiny blisters often merge together and then rupture quickly. Once they burst, the oozing fluid dries into a distinctive honey-colored crust sitting on a red, inflamed base. This golden crust is the hallmark of impetigo and what most people notice first, even though the infection technically began as those earlier small blisters.
The sores most commonly cluster around the nose and mouth, though they can appear anywhere. Over about a week, the cycle repeats: new sores form, burst, ooze, and crust over. The fluid leaking from these sores is highly contagious, and touching it can spread the infection to other parts of the body or to other people.
Bullous Impetigo Looks Different
A less common form, bullous impetigo, follows a noticeably different pattern. Instead of tiny blisters that quickly crust over, this type starts with small blisters that grow into larger, fluid-filled sacs. These larger blisters contain clear or yellowish fluid that gradually turns cloudy or dark. Unlike the classic form, the skin around these blisters typically doesn’t turn red or swell.
When the blisters eventually rupture, they leave behind a red base with a ring of peeling skin around the edges. Crucially, bullous impetigo does not produce the honey-colored crust that defines the common form. This type shows up more often on the trunk of the body and is most frequently seen in infants and young children. It’s caused specifically by staph bacteria that produce a toxin targeting the upper layers of skin, which is why the blisters grow larger and behave differently.
When Impetigo Goes Deeper
A more serious form called ecthyma develops when the infection pushes past the skin’s outer layer and into deeper tissue. The sores appear as “punched out” ulcers with dark, violet-colored edges. The crusts covering these ulcers can be honey-colored like the common form or brown-black. Ecthyma is painful in a way that typical impetigo often isn’t, and the deeper ulcers can leave scars. This form is more likely to develop when impetigo goes untreated or when someone’s immune system is already compromised.
How It Spreads
Impetigo is one of the most contagious skin infections. The bacteria spread through direct skin-to-skin contact with an infected person’s sores or through shared items like towels, clothing, and bedding that have touched the infected fluid. Children pass it easily in daycare and school settings, where close physical contact is constant and personal items get shared.
The infection also spreads on your own body. Touching or scratching one sore and then touching another area of skin can seed new lesions. This is why impetigo patches often multiply and appear in clusters rather than staying in one spot. Covering sores with bandages helps limit this kind of self-spreading and reduces the risk of passing it to others.
When You’re Contagious
A person with impetigo remains contagious as long as the sores are oozing and haven’t been treated. Once antibiotic treatment begins, the contagious window closes quickly. The CDC advises that people with impetigo can return to school or work at least 12 hours after starting antibiotics. For higher-risk situations, such as healthcare workers or outbreak settings, waiting at least 24 hours is recommended.
Without treatment, the sores can continue to spread for weeks. The infection sometimes resolves on its own over two to three weeks, but during that entire period, the person can transmit the bacteria to others. This is one of the main reasons treatment is recommended even for mild cases: it shortens the window during which the infection can jump to siblings, classmates, or anyone in close contact.
What Makes Some People More Vulnerable
Warm, humid conditions favor impetigo because sweat and moisture soften the skin barrier and create a friendlier environment for bacterial growth. The infection peaks during summer months in temperate climates and is a year-round concern in tropical regions. Crowded living conditions also raise the risk, since more people sharing space means more opportunities for skin-to-skin contact and shared surfaces.
Children between ages two and five are the most commonly affected group, largely because of behavioral factors: frequent minor injuries, nose-picking, close play with other kids, and less consistent hand hygiene. Existing skin conditions also set the stage. Eczema, chickenpox, poison ivy, or any condition that causes itching and scratching creates exactly the kind of skin breaks that bacteria exploit. If you or your child has a condition that disrupts the skin barrier, keeping those areas clean and moisturized reduces the chance of impetigo taking hold.