These are bacterial infections of the skin that usually begin from a scratch or bug bite and progress to a red nodule that fills with pus. Boils are superficial infections with a thin layer of skin over fluid; abscesses are generally larger and deeper with redness and painful swelling over an area filled with pus. Cellulitis is an infection within the skin and the area just beneath it; the skin is red and tender. The area of cellulitis can spread quickly.