  Recent years has witnessed an increase in technologies that use speech for
the sensing of the health of the talker. This survey paper proposes a general
taxonomy of the technologies and a broad overview of current progress and
challenges. Vocal biomarkers are often secondary measures that are
approximating a signal of another sensor or identifying an underlying mental,
cognitive, or physiological state. Their measurement involve disturbances and
uncertainties that may be considered as noise sources and the biomarkers are
coarsely qualified in terms of the various sources of noise involved in their
determination. While in some proposed biomarkers the error levels seem high,
there are vocal biomarkers where the errors are expected to be low and thus are
more likely to qualify as candidates for adoption in healthcare applications.
