If you need to define a word in your paper, the definition usually comes from a glossary in the back of a book. To properly attribute this definition you will need to cite it according to the American Psychological Association (APA) style guidelines.
Encephalitis is inflammation of the brain tissue. It's most often caused by viral infections. In some cases, bacterial infections can also cause encephalitis.
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.