The bacterium Streptococcus is the most common infectious organism associated with ecthyma. Staphylococcus bacteria can also cause ecthyma. The infection may start at the site of an injury, such as a scratch or insect bite, and is often found on the legs.

An ecthyma may begin with a pus-filled blister, similar to that seen in impetigo. However, the infection goes through the outer layer of skin (epidermis) and into the deeper layer (dermis). An ulcer with raised borders develops. It is covered by a hard crust. Unlike impetigo, ecthyma can sometimes result in scarring.