Brain infections can be caused by viruses, bacteria, fungi, or, occasionally, protozoa or parasites.
Encephalitis is most commonly due to viruses, such as herpes simplex Herpes Simplex Virus (HSV) Infections Herpes simplex viruses (human herpesviruses types 1 and 2) commonly cause recurrent infection affecting the skin, mouth, lips, eyes, and genitals. Common severe infections include encephalitis... read more , herpes zoster Herpes Zoster Herpes zoster is infection that results when varicella-zoster virus reactivates from its latent state in a posterior dorsal root ganglion. Symptoms usually begin with pain along the affected... read more , cytomegalovirus Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more , or West Nile virus.
Infections such as progressive multifocal leukoencephalopathy Progressive Multifocal Leukoencephalopathy (PML) Progressive multifocal leukoencephalopathy (PML) is caused by reactivation of the JC virus. The disease usually occurs in patients with impaired cell-mediated immunity, particularly patients... read more (caused by the JC virus) or subacute sclerosing panencephalitis Subacute Sclerosing Panencephalitis (SSPE) Subacute sclerosing panencephalitis is a progressive, usually fatal brain disorder occurring months to usually years after an attack of measles. It causes mental deterioration, myoclonic jerks... read more (caused by the measles virus) also affect the brain; they are characterized by a long incubation and a prolonged course.
SARS-CoV-2 infection COVID-19 COVID-19 is a respiratory illness caused by the novel coronavirus SARS-CoV-2. Infection may be asymptomatic or have symptoms ranging from mild upper respiratory symptoms to acute respiratory... read more can result in encephalopathy, although it is not clear to what extent the cause is direct viral invasion of the brain, virus-induced stroke or hemorrhage, or secondary brain insults, such as hypoxia, electrolyte abnormalities, and liver and/or kidney dysfunction.
Certain noninfectious disorders can mimic encephalitis. An example is the autoimmune condition anti-NMDA (N-methyl-d-aspartate) receptor encephalitis, which involves an autoimmune attack on neuronal membrane proteins.
Brain infections often also involve other parts of the central nervous system (CNS), 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 may cause the meninges to become inflamed (meningitis) Overview of Meningitis Meningitis is inflammation of the meninges and subarachnoid space. It may result from infections, other disorders, or reactions to drugs. Severity and acuity vary. Findings typically include... read more . Often, bacterial meningitis spreads to the brain, causing encephalitis, infecting mainly the brain parenchyma. 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, the term meningitis is usually used to refer to infection that affects mainly the meninges, and encephalitis is usually used to refer to infection that affects mainly the brain.
CNS infections may manifest as follows:
Diffuse parenchymal infection, resulting in encephalitis, sometimes affecting specific areas on the brain
Inflammation of the brain secondary to meningeal infections Overview of Meningitis Meningitis is inflammation of the meninges and subarachnoid space. It may result from infections, other disorders, or reactions to drugs. Severity and acuity vary. Findings typically include... read more or parameningeal infections
Focal or multifocal infection (eg, due to a brain abscess, empyema, or fungal or parasitic brain infections such as neurocysticercosis Cysticercosis Taenia solium infection (taeniasis) is an intestinal infection with adult tapeworms that follows ingestion of contaminated pork. Adult worms may cause mild gastrointestinal symptoms or passage... read more caused by Taenia solium Taenia Solium (Pork Tapeworm) Infection and Cysticercosis Taenia solium infection (taeniasis) is an intestinal infection with adult tapeworms that follows ingestion of contaminated pork. Adult worms may cause mild gastrointestinal symptoms or passage... read more )
HIV infection Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more and prion diseases Overview of Prion Diseases Prion diseases are progressive, fatal, and untreatable degenerative brain disorders. Prominent types of prion diseases include Creutzfeldt-Jakob disease (CJD), the prototypic example (usually... read more can also affect the brain diffusely.
Brain involvement may also be a manifestation of postinfectious mechanisms, such as acute disseminated encephalomyelitis.
Bacteria and other infectious organisms can reach the brain and meninges in several ways:
Penetrating head wounds (including neurosurgical procedures)
Direct extension of cranial infections (eg, sinusitis Sinusitis Sinusitis is inflammation of the paranasal sinuses due to viral, bacterial, or fungal infections or allergic reactions. Symptoms include nasal obstruction and congestion, purulent rhinorrhea... read more , osteomyelitis Osteomyelitis Osteomyelitis is inflammation and destruction of bone caused by bacteria, mycobacteria, or fungi. Common symptoms are localized bone pain and tenderness with constitutional symptoms (in acute... read more )
Many symptoms and signs of brain infections (eg, acute or subacute obtundation, seizures, focal neurologic deficits, signs of increased intracranial pressure) can develop in various brain infections (eg, brain abscess, intracranial epidural abscess, subdural empyema); thus neuroimaging is frequently necessary to differentiate among the infections.