AIDS dementia complex (ADC), sometimes called HIV-associated dementia (HAD), is different than other types of AIDS-related illnesses. Most AIDS-related illnesses are caused by other infections (such as bacteria, fungi, and other viruses). ADC is one of the only illnesses that can be caused directly by HIV.
HIV can pass into the brain. In fact, some studies show that HIV enters the brain in as few as two days after the virus first enters the body. HIV can damage nerve cells in the brain, although researchers don't totally understand how this happens.
ADC can happen at any T-cell count. However, it is much more likely to occur when the T-cell count falls below 200. This is because the immune system plays a major role in protecting nerves in the brain. If the immune system becomes suppressed, HIV and other organisms can damage these nerves and affect the way the brain works.
It has been estimated that between 20% and 35% of all HIV-positive people will eventually develop some symptoms of ADC. However, the number of HIV-positive people with ADC is much lower today, thanks to the availability of powerful anti-HIV drug therapy.