Apparatus and methods are described for use with feces of a subject that is disposed within a toilet bowl (23), and an output device (32). One or more light sensors (60, 62, 64, 66) receive light from the toilet bowl, while the feces are disposed within the toilet bowl. A computer processor (44) analyzes the received light, and, in response thereto, determines that there is a presence of blood within the feces, and determines a source of the blood from within the subject's gastrointestinal tract. The computer processor (44) generates an output on the output device (32), at least partially in response thereto. Other applications are also described.
