An inflammatory disease of the brain caused by a virus that either has invaded the brain, or a virus appearing elsewhere in the body that has caused a sensitivity reaction in the brain. Encephalitis infects the brain tissue itself and has serious consequences.

Among the many forms of encephalitis are those that occur seasonally, those that affect animals, and a form that is carried by a mosquito. Viruses that have been directly implicated in causing encephalitis include the arbovirus, echovirus, poliovirus, and the herpes simplex virus. Encephalitis occurs as a complication of, for example, chicken pox, polio, and vaccinia, which is a cowpox virus used in smallpox vaccinations, as well as the common flu virus. Other implicated causes include the herpes simplex virus, responsible for the common cold sore, eczema, and genital herpes; the measles (rubeola) virus; some of the 31 types of echoviruses that also cause a paralytic...