What is encephalitis? Encephalitis is a potentially serious disease marked by irritation and inflammation of the brain. This swelling or inflammation is called cerebral edema and can lead to the destruction of nerve cells, bleeding into the brain (intracerebral hemorrhage), and brain damage. While encephalitis can be a mild disease, people most at risk of developing a serious case of encephalitis include infants, young children, the elderly, and people with weakened immune systems. Encephalitis is rare in the United States. Encephalitis is most often caused by viral infections. You can be exposed to viruses and other pathogens that cause encephalitis in various ways. Viruses, bacteria and parasites can be transmitted by breathing in air droplets from an infected person, swallowing contaminated food or drink, or making person-to-person contact. Encephalitis can result from viruses that cause childhood infections, such as measles and mumps. These illnesses used to be very common, but are less frequent now because of childhood immunizations. Pathogens can also be transmitted by a bite from a rabid animal or from a tick, mosquito, or other blood-sucking insect. Once any of these pathogens enter your body, they can spread to the bloodstream where they are carried to the nervous system. In the nervous system, they multiply and cause infection and inflammation of the brain. Other causes of encephalitis include autoimmune disorders, allergic reactions to vaccines, and certain cancers. Typical symptoms of encephalitis include a low-grade fever, mild headache, neck stiffness, low energy, and poor appetite. Treatment of encephalitis varies depending on the type and severity of encephalitis. Almost all people with mild cases of encephalitis will recover, but more severe cases of encephalitis can be life threatening if they are not quickly diagnosed and treated. Encephalitis is a disease that can cause serious complications, such as problems with vision, memory and speech, and can be life threatening in some cases. Seek prompt medical care if you have been exposed to someone who has encephalitis. Seek immediate medical care (call 911) if you, or someone you are with, have symptoms of encephalitis, such as fever, severe headache, change in consciousness, hallucinations, seizure, muscle weakness or paralysis, changes in mental function, and changes in speech, hearing or vision.