Encephalotoxin produced by activated mononuclear phagocytes is present in individuals having neurological disease including neurodegenerative and neuro-inflammatory diseases, such as Alzheimer's disease (AD), HIV-1-associated dementia (HAD), Creutzfeldt-Jakob Disease, Mild Cognitive Impairment, prion disease, minor cognitive/motor dysfunction, acute stroke, acute trauma, or neuro-AIDS. Biochemical detection of encephalotoxin according to the methods of the invention will allow diagnosis of neurological disease in early, presymptomatic stages, thereby allowing early intervention in disease progression as well as identification of subjects or populations at risk for developing neurodegenerative disease. The methods of the invention also provide a mechanism for monitoring progression and treatment of neurological disease.