Sometimes a bacterial or viral infection that enters the brain can cause dementia. For example, when HIV develops into AIDS, it may cause a person to experience problems with memory and concentration, a loss of motivation, and decreased interest in things that were previously enjoyed. The presence of other AIDS symptoms, along with an HIV (blood) test, will help a physician determine whether the dementia is HIV related.

Creutzfeldt-Jakob disease is another example. It's very rare -- one case per million people per year. ("Mad cow disease" is one form.) Creutzfeldt-Jakob disease is caused by transmission of a prion (an infectious protein). The prions infect and subsequently destroy the brain's nerve cells. Unlike dementia caused by Alzheimer's, memory problems and behavioral changes caused by Creutzfeldt-Jakob disease progress very quickly.

A physician may diagnose Creutzfeldt-Jakob disease through a medical history, a neurological exam, an electroencephalogram (or EEG, which tracks the electrical activity in the brain), a brain scan (an MRI may be especially useful), and a cerebral spinal fluid analysis (spinal tap). Only an autopsy can definitively confirm the diagnosis; the destruction of brain cells is apparent by holes in the brain tissue.