Cellulitis

Cellulitis begins with a breach in the skin's barrier

Intact skin normally excludes bacteria. A break (trauma, tinea pedis, eczema, a venous ulcer, an injection site) allows skin flora, chiefly Staphylococcus aureus and Streptococcus pyogenes, to invade the dermis and subcutaneous tissue. Because there is no discrete collection of pus at this stage, unlike an abscess, the infection spreads diffusely along tissue planes, which is why cellulitis produces an ill-defined, spreading erythema rather than a localised lump.