Encephalitis (en-sef-uh-LIE-tis) is inflammation of the brain. There are several causes, but the most common is a viral infection. Encephalitis often causes only mild flu-like signs and symptoms — such as a fever or headache — or no symptoms at all. Sometimes the flu-like symptoms are more severe. Symptoms of a brain abscess headache – which is often severe, located in a single section of the head and cannot be relieved with painkillers. changes in mental state – such as confusion or irritability. problems with nerve function – such as muscle weakness, slurred speech or paralysis on one side of the body.