Definition:

Encephalitis is an inflammatory process of the brain parenchyma that usually is an acute process, but may be a post-infectious encephalomyelitis, a chronic degenerative disease, or a slow viral infection

• Encephalitis may be diffuse or localized.
• Organisms cause encephalitis by one of two mechanisms:

1. Direct infection of the brain parenchyma
2. Immune-mediated response in the CNS (usually begins several days after the appearance of extra-neural manifestations of the infection).