Cellulitis is an infection of the skin and underlying tissues that can affect any area of the body. Not to be confused with cellulite - the cottage-cheese-like, lumpy fat often found on the hips, thighs, and buttocks, primarily of women - cellulitis begins in an area of broken skin, like a cut or scratch, allowing bacteria to invade and spread, causing inflammation, which includes pain, swelling, warmth, and redness.