Cellulitis is an acute inflammation of the connective tissue of the skin, caused by infection with staphylococcus, streptococcus or other bacteria. The skin normally has many types of bacteria living on it, but intact skin is an effective barrier that keeps these bacteria from entering and growing within the body. When there is a break in the skin, however, bacteria can enter the body and grow there, causing infection and inflammation. The skin tissues in the infected area become red, hot, irritated and painful.

Cellulitis is most common on the face and lower legs, although skin on other areas of the body may sometimes be involved.