Causes :
Impetigo is a bacterial infection. Your skin surface and the inside of your nose are normally home to large numbers of “friendly” or commensal bacteria that help protect you from disease-causing bacteria such as Staphylococcus aureus and Streptococcus pyogenes.
Your commensal bacteria work to keep down the population of pathogenic bacteria by producing substances that are toxic to the pathogens, depriving them of nutrients, among other measures.
But strains of these staph or strep bacteria can take advantage of a break in the skin from a cut, scratch, insect bite, or rash to invade and colonize, causing impetigo.
The bacteria can also colonize and cause an infection on normal skin (12). It’s not known exactly why this happens.
Within about 10 days of bacteria colonization, impetigo blisters appear (12). The way it works is that the Staphylococcus aureus and Streptococcus pyogenes bacteria produce toxins that break apart your top skin layers, causing blisters to form.
In many cases, the bacteria are already on site, waiting for an opportunity to colonize:
Staphylococcus aureus and Streptococcus pyogenes bacteria are normally carried in the nose by between 20 and 50 percent of the general population.
Further, about 10 to 20 percent of healthy people have Staphylococcus aureus bacteria in their perineum (the area between the genitals and the anus).