Impetigo is a skin infection caused by bacteria. Usually the cause is staphylococcal (staph) but sometimes streptococcus (strep) can cause it, too. It is most common in children between the ages of two and six. It usually starts when bacteria get into a break in the skin, such as a cut, scratch or insect bite. Symptoms start with red or pimple-like sores surrounded by red skin. These sores can be anywhere, but usually they occur on your face, arms and legs. The sores fill with pus, then break open after a few days and form a thick crust. They are often itchy, but scratching them can spread the sores. Impetigo can spread by contact with sores or nasal discharge from an infected person. You can treat impetigo with antibiotics.