domains, especially in learning, information processing and concentration/attention. The cognitive impairment is ascertained by medical history, mental status examination or neuropsychological testing. Cognitive impairments identified in 1 interfere markedly with day-to-day functioning. Cognitive impairments identified in 1 are present for at least one month. Cognitive impairments identified in 1 do not meet the criteria for delirium, or if delirium is present, dementia was diagnosed when delirium was not present. No evidence of another, pre-existing cause that could explain the dementia (e.g. another CNS infection, CNS neoplasm, cerebrovascular disease, pre-existing neurological disease, severe substance abuse compatible with CNS disorder. While the progression of dysfunction is variable, it is regarded as a serious complication and untreated can progress to a fatal outcome. Diagnosis is made by neurologists who carefully rule out alternative diagnoses. This routinely requires a careful neurological examination, brain scans (MRI or CT scan) and a lumbar puncture to evaluate the cerebrospinal fluid. No single test is available to confirm the diagnosis, but the constellation of history, laboratory findings and examination can reliably establish the diagnosis when performed by experienced clinicians. The amount of virus in the brain does not correlate well with the degree of dementia, suggesting that secondary mechanisms are also important in the manifestation of HAD. HAD stage characteristics Stage 0 (Normal) Normal Mental and Motor Function Stage 0.5 (Subclinical) Minimal symptoms of cognitive or motor dysfunction characteristic of HAD, or mild signs (snout response, slowed extremity movements), but without impairment of work or capacity to perform activities of daily living (ADL). Gait and strength are normal. Stage 1 (Mild) Evidence of functional intellectual or motor impairment characteristic of HAD, but able to perform all but the more demanding aspects of work or ADL. Can walk without assistance. Stage 2 (Moderate) Cannot work or maintain the more demanding aspects of daily life, but able to perform basic activities of self care. Ambulatory, but may require a single prop. Stage 3 (Severe) Major intellectual incapacity - cannot follow news or personal events, cannot sustain complex conversation, considerable slowing of all output. And/or motor disability - cannot walk unassisted, requiring walker or personal support, usually with slowing and clumsiness of arms as well. Stage 4 (End Stage) Nearly vegetative. Intellectual and social comprehension and responses are at a rudimentary level. Nearly or absolutely mute. Paraparetic or paraplegic with urinary incontinence and fecal incontinence. Neuroimaging studies A study by Melrose