Impetigo in adults is caused by bacteria, most commonly Staphylococcus aureus (staph) or Streptococcus pyogenes (strep), entering the skin through a break in its surface. While impetigo is far more common in children aged 2 to 5, adults develop it when something compromises their skin barrier or immune defenses, giving these bacteria an opening.
The Bacteria Behind It
Two bacteria cause nearly all impetigo cases. Staph is the more frequent culprit in adults and is responsible for both forms of the infection. Strep can cause impetigo on its own or alongside staph. In many cases, both bacteria are present in the same lesion.
These bacteria are common. Staph lives on the skin or in the nose of roughly a third of the population without causing problems. The issue isn’t the presence of the bacteria; it’s what lets them get past the skin’s outer layer. On intact skin, the cell receptors these bacteria need to latch onto (called fibronectin receptors) are hidden. When the skin is disrupted, those receptors become exposed, and bacteria can colonize the area. Once that happens, impetigo lesions typically develop within one to two weeks.
How Bacteria Get Through the Skin
Healthy, unbroken skin is a strong barrier against impetigo. Adults almost always develop it after something damages or weakens that barrier. The list of potential entry points is long:
- Cuts, scrapes, and abrasions from sports, manual labor, or everyday activity
- Insect bites, which create small wounds and trigger scratching
- Existing skin conditions like eczema (atopic dermatitis), fungal infections, or psoriasis
- Other infections such as scabies, cold sores (herpes simplex), or chickenpox
- Burns, surgical wounds, or radiation therapy that compromise the skin’s integrity
Scratching is one of the most common triggers. Any condition that makes you itch, from eczema to a mosquito bite, creates micro-tears in the skin that bacteria exploit. Adults with eczema are at particularly high risk because staph colonizes their skin at dramatically higher rates. Studies show 60 to 90 percent of people with atopic dermatitis carry staph on their skin, compared to roughly 30 percent of the general population. That heavy colonization means any small break in the skin is far more likely to become infected.
Two Types, Two Different Causes
Impetigo comes in two forms, and they have slightly different bacterial origins.
Non-bullous impetigo is the more common type, producing the characteristic honey-colored crusted sores. It can be caused by staph, strep, or both together. This is the form most adults encounter.
Bullous impetigo produces larger, fluid-filled blisters and is caused exclusively by toxin-producing strains of staph. The toxins these bacteria release break down the proteins that hold skin cells together, creating the blistering effect. Bullous impetigo is less common in adults but does occur, particularly in people with weakened immune systems.
Why Some Adults Are More Vulnerable
Certain health conditions make adults significantly more prone to impetigo. Diabetes is a major risk factor because elevated blood sugar impairs the immune system’s ability to fight bacterial infections and can reduce blood flow to the skin. A weakened immune system from any cause, whether from medications like corticosteroids, chemotherapy, or conditions like HIV, increases susceptibility.
Older adults face higher risk as well. Aging skin is thinner, drier, and slower to heal, making it easier for bacteria to penetrate and harder for the body to clear an infection quickly. Recent antibiotic use can also shift the balance of normal skin bacteria in ways that allow staph or strep to gain a foothold more easily.
Environment and Lifestyle Factors
Hot, humid weather creates conditions where impetigo-causing bacteria thrive. High temperature and humidity modify the skin’s normal bacterial balance and promote colonization by staph and strep. This is why impetigo cases spike in summer months and are more common in tropical climates year-round.
Close-contact activities and shared environments raise risk as well. Contact sports, gym equipment, and shared towels or razors can spread staph from person to person. The bacteria pass most commonly through direct hand contact, entering through any small wound on the recipient’s skin. Adults who participate in activities where cuts and scrapes are common, from rugby to construction work, face higher exposure.
When Impetigo Goes Deeper
Left untreated, impetigo can progress to a deeper infection called ecthyma, sometimes referred to as “deep impetigo.” While standard impetigo affects only the outermost skin layer, ecthyma penetrates into the deeper layers, creating painful ulcers that can leave permanent scars. Ecthyma is most often caused by strep, sometimes in combination with staph.
Adults with diabetes or weakened immune systems are more prone to ecthyma and to the infection spreading to other parts of the body. This is one reason impetigo in adults warrants attention even though it’s often considered a mild childhood illness. In children, it typically resolves without complications. In adults, the underlying conditions that allowed the infection to develop in the first place can also make it harder to clear and more likely to cause lasting damage.
How It Spreads
Impetigo is highly contagious. The bacteria spread through direct skin-to-skin contact with an infected person’s sores or through touching contaminated objects like towels, clothing, or bedding. You can also spread it to other parts of your own body by touching or scratching a lesion and then touching unbroken skin elsewhere. This “auto-inoculation” is why impetigo sores often appear in clusters or spread to nearby areas.
Adults living with someone who has impetigo, especially a child, are at risk of contracting it themselves if they have any of the predisposing factors: broken skin, a chronic skin condition, or a compromised immune system. The combination of exposure to the bacteria and a vulnerable skin barrier is what ultimately causes the infection to take hold.