Cellulitis is a bacterial skin infection that causes redness, swelling, and pain in the infected area of the skin. It usually affects the lower legs, but it can occur anywhere on the body. The bacteria that cause cellulitis often live harmlessly on the skin, but they can lead to an infection if they get into a break in the skin, such as a cut, scratch, insect bite, or surgical wound. Sometimes the break in the skin is too small to notice, and the trigger for the infection is unknown. The most common bacteria that cause cellulitis include streptococcus and staphylococcus. Other factors that increase the risk of cellulitis include poor circulation, obesity, diabetes, smoking, and skin conditions that make you itchy and more likely to scratch. If left untreated, cellulitis can spread to the lymph nodes and bloodstream and rapidly become life-threatening.