Infections of the brain can be caused by viruses, bacteria, fungi, or, occasionally, protozoa or parasites. Another group of brain disorders, called spongiform encephalopathies, are caused by abnormal proteins called prions Overview of Prion Diseases Prion diseases are rare progressive, fatal, and currently untreatable degenerative disorders of the brain (and rarely of other organs) that result when a protein changes into an abnormal form... read more .
Infections of the brain often also involve other parts of the central nervous system, including the spinal cord. The brain and spinal cord are usually protected from infection, but when they become infected, the consequences are often very serious.
Infections can cause inflammation of the brain (encephalitis Encephalitis Encephalitis is inflammation of the brain that occurs when a virus directly infects the brain or when a virus, vaccine, or something else triggers inflammation. The spinal cord may also be involved... read more ). Viruses are the most common causes of encephalitis. Infections can also cause inflammation of the layers of tissue (meninges) that cover the brain and spinal cord—called meningitis Introduction to Meningitis Meningitis is inflammation of the layers of tissue that cover the brain and spinal cord (meninges) and of the fluid-filled space between the meninges (subarachnoid space). Meningitis can be... read more . Often, bacterial meningitis spreads to the brain itself, causing encephalitis. Similarly, viral infections that cause encephalitis often also cause meningitis. Technically, when both the brain and the meninges are infected, the disorder is called meningoencephalitis. However, infection that affects mainly the meninges is usually called meningitis, and infection that affects mainly the brain is usually called encephalitis.
Usually in encephalitis and meningitis, infection is not confined to one area. It may occur throughout the brain or within meninges along the entire length of the spinal cord and over the entire brain.
But in some disorders, infection is confined to one area (localized) as a pocket of pus, called an empyema or an abscess, depending on where it is located:
Empyemas Intracranial Epidural Abscess and Subdural Empyema An intracranial epidural abscess is a pocket of pus that develops between the skull and the top layer of tissues (dura mater) covering the brain. A subdural empyema is a pocket of pus that develops... read more form in an existing space in the body, such as the space between the tissues that cover the brain (meninges) or the lungs.
Abscesses Abscess of the Brain A brain abscess is a pocket of pus in the brain. An abscess may form in the brain when bacteria from an infection elsewhere in the head or in the bloodstream or from a wound enter the brain... read more , which resemble boils, can form anywhere in the body, including within the brain.
Fungi (such as aspergilli Aspergillosis Aspergillosis is an infection, usually of the lungs, caused by the fungus Aspergillus. A ball of fungus fibers, blood clots, and white blood cells may form in the lungs or sinuses. People... read more 
), protozoa (such as Toxoplasma gondii Toxoplasmosis Toxoplasmosis is infection caused by the protozoan parasite Toxoplasma gondii. Infection occurs when people unknowingly ingest toxoplasma cysts from cat feces or eat contaminated meat... read more ), and parasites (such as Taenia solium, the pork tapeworm Tapeworm Infection Tapeworm infection of the intestine occurs mainly when people eat raw or undercooked contaminated pork, beef, or freshwater fish or, for the dwarf tapeworm, contaminated food or water. Adult... read more 
) may cause cysts to form in the brain. These localized brain infections consist of a cluster of organisms enclosed in a protective wall.
Sometimes a brain infection, a vaccine, cancer, or another disorder triggers a misguided immune reaction, causing the immune system to attack normal cells in the brain (an autoimmune reaction Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers an autoimmune disorder is not known. Symptoms vary depending... read more ). As a result, the brain becomes inflamed. This disorder is called postinfectious encephalitis.
Bacteria and other infectious organisms can reach the brain and meninges in several ways:
By being carried by the blood
By entering the brain directly from the outside (for example, through a skull fracture or during surgery on the brain)
By spreading from nearby infected structures, such as the sinuses or middle ear
