Brain abscesses are usually polymicrobial in nature.  Swelling and irritation (inflammation) develop in response to this infection. Infected brain cells, white blood cells, live and dead bacteria, and fungi collect in an area of the brain. Tissue forms around this area and creates a mass.  While this immune response can protect the brain by isolating the infection, it can also do more harm than good. Infected material can block the blood vessels of the brain. Although underlying pathology (tumor, blood etc.) can sometimes be a nidus for infection, the majority of cases occur in a previously healthy brain.
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