Encephalitis is defined as inflammation of the brain. This definition means encephalitis is different from meningitis, which is defined as inflammation of the layers of tissue, or membranes, covering the brain. Unfortunately, in some people, both of the diseases may coexist and lead to a more complex diagnosis and treatment plan; in addition, both conditions share many of the same symptoms so they may be difficult to distinguish. There are many causes of encephalitis: viruses, bacteria, parasites, chemicals, and even autoimmune reactions. This article is designed to discuss general features of encephalitis; it is not designed to be all inclusive as book chapters have been written on individual causes. The reader wanting more information than is present in this introduction is urged to click on the links provided and to check the references provided at the end of this article.In clinical practice, most doctors consider encephalitis to be a viral illness. Viruses such as those responsible for causing cold sores, mumps, measles, and chickenpox can also cause encephalitis; they will not be further discussed in this article because their major disease manifestations, symptoms, and complications are detailed in other articles. Major causes of viral encephalitis are herpes viruses and the arboviruses. Arboviruses are spread by insects such as mosquitoes and ticks. The equine (meaning horse), West Nile, Japanese, La Crosse, and St. Louis encephalitis viruses are all mosquito-borne arboviruses. Although viruses are the most common source of infection, bacteria, fungi, parasites, chemicals, and autoimmune reactions can also be responsible for encephalitis. However, current data suggest that these are far less common than viral infections as causes of encephalitis.Viral encephalitis resembles the flu in terms of its symptoms and usually lasts for two to three weeks. It can vary from mild to life-threatening and even cause death. Most people with a mild infection can recover fully. Those with a more severe infection can recover although they may have damage to their nervous system. This damage can be permanent. Some other general features of viral encephalitis are as follows:Age, season, geographic location, regional climate conditions, and the strength of the person's immune system play a role in development of the disease and severity of the illness.Herpes simplex (the virus causing cold sores) remains the most common virus involved in encephalitis in the United States and throughout the world. These viruses are usually transmitted from person to person.In the United States, there are five main encephalitis-causing viruses spread by mosquitoes: West Nile, Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), La Crosse, and St. Louis encephalitis. Two types of Powassan viruses, an infrequent cause of encephalitis, are transmitted by at least two types of ticks.Venezuelan equine encephalitis is found in South America. It can be a rare cause of encephalitis in the southwestern United States, particularly Texas. The infection is very mild, and nervous system damage is rare.Japanese encephalitis virus is the most common arbovirus in the world (virus transmitted by blood-sucking mosquitoes or ticks) and is responsible for 50,000 cases and 15,000 deaths per year worldwide. Most of China, Southeast Asia, and the Indian subcontinent are affected.