Brain infections can be caused by viruses, bacteria, fungi, or, occasionally, protozoa or parasites.
Infections such as progressive multifocal leukoencephalopathy (caused by the JC virus) or subacute sclerosing panencephalitis (caused by the measles virus) also affect the brain; they are characterized by a long incubation and a prolonged course.
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). 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 or parameningeal infections
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: