Cellulitis

Cellulitis is an acute bacterial infection of the skin, but not of the top layer: it inflames the deep dermis, the thick layer that lies beneath the surface, and the surrounding subcutaneous tissue, the layer of fat around it. By definition, in cellulitis there is no abscess and no pus coming out.

It starts at a point of entry. The skin works as a barrier that keeps out the normal flora, the microbes that live on top without doing harm, and other microbes as well. When that barrier breaks, those bacteria pass into the dermis and the subcutaneous tissue, which until then were protected.

What you see on the outside, the redness, the warmth, the swelling and the tenderness to touch, is not produced by the bacteria alone: it comes from the body's response, with cytokines, the immune alarm signals, and neutrophils, the white blood cells that show up. And it has a close cousin it is often confused with: erysipelas, which is more superficial and has sharp borders. Cellulitis runs deeper and is more poorly demarcated. The blurry border is the clue: the inflamed part runs underneath.