Apparatus for use with feces of a subject that are disposed within a toilet bowl, and an output device, the apparatus comprising: one or more light sensors (24, 60, 62) that are configured to receive light from the toilet bowl, while the feces are disposed within the toilet bowl; and a computer processor configured to: determine that there is a presence of blood within the feces, by analyzing the received light; determine that a particular region of the subject's gastrointestinal tract is a source of the blood, at least partially by: measuring an intensity of a first spectral component, within the received light, the first spectral component being centered around a wavelength of between 590 nm and 1000 nm, or a wavelength of between 480 nm and 520 nm; measuring an intensity of a second spectral component, within the received light, that is centered around a wavelength of between 520 and 590 nm; and normalizing the measured intensity of the first spectral component with respect to the measured intensity of the second spectral component; and generate an output on the output device, at least partially in response thereto, wherein the computer processor is further configured to determine that the particular region of the subject's gastrointestinal tract is the source of the blood, by analyzing a spatial distribution of the blood within the feces. one or more light sensors (24, 60, 62) that are configured to receive light from the toilet bowl, while the feces are disposed within the toilet bowl; and a computer processor configured to: determine that there is a presence of blood within the feces, by analyzing the received light; determine that a particular region of the subject's gastrointestinal tract is a source of the blood, at least partially by: measuring an intensity of a first spectral component, within the received light, the first spectral component being centered around a wavelength of between 590 nm and 1000 nm, or a wavelength of between 480 nm and 520 nm; measuring an intensity of a second spectral component, within the received light, that is centered around a wavelength of between 520 and 590 nm; and normalizing the measured intensity of the first spectral component with respect to the measured intensity of the second spectral component; and generate an output on the output device, at least partially in response thereto, wherein the computer processor is further configured to determine that the particular region of the subject's gastrointestinal tract is the source of the blood, by analyzing a spatial distribution of the blood within the feces. determine that there is a presence of blood within the feces, by analyzing the received light; determine that a particular region of the subject's gastrointestinal tract is a source of the blood, at least partially by: measuring an intensity of a first spectral component, within the received light, the first spectral component being centered around a wavelength of between 590 nm and 1000 nm, or a wavelength of between 480 nm and 520 nm; measuring an intensity of a second spectral component, within the received light, that is centered around a wavelength of between 520 and 590 nm; and normalizing the measured intensity of the first spectral component with respect to the measured intensity of the second spectral component; and measuring an intensity of a first spectral component, within the received light, the first spectral component being centered around a wavelength of between 590 nm and 1000 nm, or a wavelength of between 480 nm and 520 nm; measuring an intensity of a second spectral component, within the received light, that is centered around a wavelength of between 520 and 590 nm; and normalizing the measured intensity of the first spectral component with respect to the measured intensity of the second spectral component; and generate an output on the output device, at least partially in response thereto, wherein the computer processor is further configured to determine that the particular region of the subject's gastrointestinal tract is the source of the blood, by analyzing a spatial distribution of the blood within the feces.