Clinical features are pain, swelling, erythema and warmth. Cellulitis can be non-purulent or purulent (abscesses). If patient is very ill or has pain out of proportion to appearance, consider necrotizing fasciitis.
Neonatal mastitis, omphalitis, dacryocystitis are usually due to Staphylococcus aureus
Erythema migrans (Lyme Disease) lesions are usually large (>5cm), and not tender (though can be mildly pruritic).
Herpes (vesicles or shallow ulcers).
Insect bites or urticaria which are very pruritic, not tender.