Encephalitis
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.