While breast-feeding the baby, the skin of the breast may become irritated and inflamed. The skin may fissure, predisposing to infection. Acute mastitis typically involves just one breast and is most often caused by Staphylococcus aureus, though other bacterial organisms such as streptococci can produce this condition, with neutrophilic infiltrates microscopically. If untreated by antibiotic therapy, spread of infection and abscess formation can occur.