Encephalitis (en-sef-uh-LIE-tis) is inflammation of the brain. There are several causes, including viral infection, autoimmune inflammation, bacterial infection, insect bites and others. When inflammation is caused by an infection in the brain, it's known as infectious encephalitis. And when it's caused by your own immune system attacking the brain, it's known as autoimmune encephalitis. Sometimes there is no known cause. In some cases, encephalitis can be life-threatening. Prompt diagnosis and treatment are important because it's difficult to predict how encephalitis will affect each individual.