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Overview of Brain Infections

By

John E. Greenlee

, MD, University of Utah Health

Last full review/revision Jul 2020| Content last modified Jul 2020
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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:

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 autoimmune disorders is not known. Symptoms vary depending on... 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

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