Encephalitis (inflammation of the brain) is generally divided into two categories: primary encephalitis, which occurs when a virus invades the central nervous system (brain and spinal cord) directly; and secondary (postinfectious) encephalitis, which occurs when a virus first infects another part of the body and later enters the brain.

In addition, encephalitis may be further classified according to its cause:

• Viral encephalitis. The most common form, caused by infection from a virus. For instance, herpes simplex virus type 1 is the most common cause of encephalitis in the United States.
  
  
• Arthropod-borne encephalitis. A type of viral encephalitis that occurs after being bitten by an infected arthropod (e.g., mosquito, tick). These are further categorized according to the type of organism and/or the geographic region (e.g., Japanese encephalitis) where the diseases are more common.
  
  
• Bacterial, parasitic or roundworm encephalitis. Rare forms caused by infection with (respectively) bacteria, parasites or roundworms (from exposure to animal feces). The length of time required for incubation of these forms of encephalitis in humans varies depending on the cause and severity.
  
  
• Acute disseminated encephalomyelitis (ADEM). Typically a disease of prepubescent children, ADEM is rare in people over the age of 12 years. Signs and symptoms of ADEM are similar to those of multiple sclerosis. However, ADEM is usually associated with preceding infectious illness or immunization.

Encephalitis may also be categorized according to where it occurs in the body (e.g., cortical encephalitis only affects the brain cortex) or by accompanying conditions (e.g., purulent encephalitis is inflammation accompanied by brain abscesses). Infantile arthritis (caused by arboviruses or the herpes virus) and encephalitis periaxialis (inflammation of the white matter of the cerebrum) usually affect infants and young children.