Viral encephalitis can happen when a virus infects the brain. The most common causes are the rabies virus, Herpes Simplex Virus(HSV), the polio virus, and the measles virus. Viral encephalitis can also be caused by a "latent" virus - a virus that hides from the body's immune system the brain's nerve cells. Two examples of viruses that cause latent infection are the varicella-zoster viruses, which can hide in the brain after causing chicken pox, and the herpes simplex virus.