What Is It?
Encephalitis means inflammation of the brain. This inflammation usually is triggered by a viral infection, although sometimes it can be caused by a bacterial infection of the brain, such as Lyme disease. In some cases, symptoms are caused by direct infection of the brain. In other cases, the brain inflammation is caused by the immune system's response to the brain infection. Even if the immune system attack succeeds in eliminating the infection, it may injure the brain in the process. This is called post-infectious encephalitis.
Often, viruses that cause encephalitis also cause inflammation of the delicate tissues that cover the brain and spinal cord, which are called the meninges. This condition is meningitis. When encephalitis and meningitis occur together, it is called meningoencephalitis.
Of the many different viruses that can cause meningoencephalitis, enteroviruses (particularly coxsackievirus and echovirus) are the most common cause in the United States, particularly if the illness occurs in the summer or fall. Encephalitis also can be caused by the herpes simplex virus, which also causes cold sores and genital herpes. This type of encephalitis is less common but tends to be more severe. The mumps and measles viruses also can cause encephalitis, with mumps occurring most often in the winter or spring.