A method includes: obtaining, by a computer, subject response data including audio data, video data, or audio-video data of a subject's response to one or more stimuli presented to the subject over a defined time period; extracting at least one subject descriptor from the subject response data as a function of time, the at least one subject descriptor including data characterizing involuntary or voluntary action of the subject in response to the one or more stimuli; deriving, from the at least one subject descriptor, a first biomarker characterizing a behavior of the subject in response to the one or more stimuli, in which a value of the first biomarker is indicative of a severity of a disease in the subject; and outputting a disease severity level for the disease as a function of the value of the first biomarker.