Meningitis is an inflammation of the three membranes that cover the brain and spinal cord (the meninges). Encephalitis is an inflammation of the brain. Meningoencephalitis is an inflammation of both the brain and the meninges. These conditions are caused by a bacterial, viral, fungal, or parasitic infection. Meningitis and encephalitis can be acute or chronic, and their severity can range from mild and self-limited to fatal. Their associated inflammation and swelling increases pressure on the brain and nerve tissue. This can hinder, or permanently damage, the function of nerves and the body systems that they control.

Meningitis and encephalitis can also disrupt the blood-brain barrier that separates the brain from circulating blood and regulates the distribution of substances between the blood and cerebrospinal fluid (CSF). The blood-brain barrier helps keep large molecules, toxins, and most blood cells away from the brain. With the disruption of this barrier, white and red blood cells, immune system chemicals, toxins, increased amounts of protein, and the microorganism causing the inflammation may be found in the CSF. CSF is a clear watery liquid that normally flows freely around the brain and spinal cord. With meningitis and encephalitis, the flow of CSF may slow or become obstructed, which can increase CSF pressure, increase pressure on the brain and spinal cord, and decrease blood flow to the brain.