A method comprising: subsequent to a subject emitting feces into a toilet bowl, and without requiring any action to be performed by any person subsequent to the emission: receiving light from the toilet bowl, using one or more light sensors; and using a computer processor: detecting spectral bands within the received light, each of the spectral bands being centered around a respective wavelength of 540 nm plus/minus 5 nm, 565 nm plus/minus 5 nm, and 575 nm plus/minus 5 nm; determining a relationship between intensities of respective spectral bands, by: determining a first ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 575 nm plus/minus 5 nm; and determining a second ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 540 nm plus/minus 5 nm; in response to the relationship between intensities of the respective spectral bands, determining that there is a presence of blood within the feces; and generating an output on an output device, at least partially in response thereto. receiving light from the toilet bowl, using one or more light sensors; and using a computer processor: detecting spectral bands within the received light, each of the spectral bands being centered around a respective wavelength of 540 nm plus/minus 5 nm, 565 nm plus/minus 5 nm, and 575 nm plus/minus 5 nm; determining a relationship between intensities of respective spectral bands, by: determining a first ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 575 nm plus/minus 5 nm; and determining a second ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 540 nm plus/minus 5 nm; in response to the relationship between intensities of the respective spectral bands, determining that there is a presence of blood within the feces; and generating an output on an output device, at least partially in response thereto. detecting spectral bands within the received light, each of the spectral bands being centered around a respective wavelength of 540 nm plus/minus 5 nm, 565 nm plus/minus 5 nm, and 575 nm plus/minus 5 nm; determining a relationship between intensities of respective spectral bands, by: determining a first ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 575 nm plus/minus 5 nm; and determining a second ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 540 nm plus/minus 5 nm; determining a first ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 575 nm plus/minus 5 nm; and determining a second ratio between an intensity of the band that is centered around 565 nm plus/minus 5 nm and an intensity of the band that is centered around 540 nm plus/minus 5 nm; in response to the relationship between intensities of the respective spectral bands, determining that there is a presence of blood within the feces; and generating an output on an output device, at least partially in response thereto.