Cellulitis is a common skin infection caused by bacteria. It affects the middle layer of the skin and the tissues below. Sometimes muscle below the skin can be affected.
Its symptoms are specific enough to be useful:
Skin redness or inflammation that gets bigger as the infection spreads. A skin sore or rash that starts suddenly and grows quickly in the first 24 hours. Pain or tenderness in the affected area. Warm skin in the area of redness. A tight, glossy, stretched appearance of the skin. Fever with chills and sweating. Fatigue. Muscle aches and joint stiffness from swelling of the tissue over the joint. Nausea and vomiting.
Two things on that list are about change rather than appearance, and they are the ones that set it apart. Redness that gets bigger. A rash that grows quickly in the first 24 hours. Cellulitis is something that moves.
A wound is one of the ways it starts. Normal skin has many types of bacteria living on it, and when there is a break in the skin, these bacteria can cause an infection. Risk factors include injury or trauma with a break in the skin; skin ulcers from certain diseases, including diabetes and vascular disease; a wound from a recent surgery; and cracks or peeling skin between the toes.
There is a simple technique you can understand and ask about: your provider may mark the edges of the redness with a pen, to see if the redness goes past the marked border over the next several days. That is how spread is measured rather than guessed.
What to do about it is clear: contact your provider right away if you have symptoms of cellulitis.