HIV-negative Thais

Inclusion Criteria:

1. Age 20-69
2. Male or female
3. Negative HIV antibody at screening
4. Have the ability to understand the study and signed the informed consent form

Exclusion Criteria:

1. Head injury with loss of consciousness greater than 1 hour
2. Current or past illicit drug use or positive drug screen for amphetamine, methamphetamines, cocaine, marajuana, or narcotics
3. Severe illness within 30 days prior to entry
4. Current or recent fevers or meningeal signs.
5. History of pre-existing neurologic disease to include stroke, multiple sclerosis or autoimmune disease.
6. active major depression will be excluded as well - patients with past depresion that is controled and patients with or minor depressive symptoms will be allowed to enroll.
7. Known learning disability including dyslexia
8. Confusion or other signs and symptoms of metabolic encephalopathy or delirium
9. Focal neurological deficit on examination
10. Examining physician's concern that participant may not be able to complete visit requirements.