Abscesses are localized collections of pus.
Pyogenic organisms are the infectious agent involved with abscess formation. Empyema describes the accumulation of pus within a space or cavity, again, typically secondary to pyogenic organism infection. Epidural and subdural infections can be called either abscess or empyema 2. No organisms can be cultured, unless the empyema is accompanied by infection of leptomeninges. Space-occupying effects plus edema of brain may lead to herniation and midline shift.