The entry of germs to the brain can occur through three routes:
- Hematogenous spread (through the bloodstream) from a distant source of infection (usually nasopharyngeal).
- Contiguous infection : from infectious foci of adjacent cranial structures, such as the ear or paranasal sinuses.
- Infection by direct entry , as occurs in head injuries causing deep injuries or, less commonly, after neurosurgical intervention.
The most common bacteria isolated from brain abscesses are usually anaerobic and sometimes mixed, including anaerobic streptococci and bacteroids. Fungi, protozoa, and parasites can also cause localized intracranial infections.