A method for diagnosing meningitis, comprising the following steps: - determining a number of white blood cells in a predetermined amount of cerebrospinal fluid via detecting at least one particle in a bodily fluid, with a detection system comprising a light source, a transparent substrate and a photodetector array, the transparent substrate being positioned between the light source and the photodetector array, the bodily fluid being the cerebrospinal fluid and the detected particles being the white blood cells contained in said cerebrospinal liquid, the detecting comprising the following steps: + placing the bodily fluid as a droplet on the transparent substrate, + illuminating the droplet with the light source, + acquiring several successive images (I n, I n+1) of the droplet with the photodetector array, each image (I n) being formed by radiation transmitted through the illuminated droplet and including at least one elementary diffraction pattern, each elementary diffraction pattern corresponding to waves diffracted by a particle during the illumination of the droplet, + identifying, from the acquired images (I n, I n+1), mobile elementary diffraction patterns, + counting moving particles within the droplet, from the thereby identified mobile elementary diffraction patterns, and - diagnosing meningitis if the number of detected white blood cells in the predetermined amount of cerebrospinal fluid is greater than a predetermined threshold value, by counting only the moving white blood cells. - determining a number of white blood cells in a predetermined amount of cerebrospinal fluid via detecting at least one particle in a bodily fluid, with a detection system comprising a light source, a transparent substrate and a photodetector array, the transparent substrate being positioned between the light source and the photodetector array, the bodily fluid being the cerebrospinal fluid and the detected particles being the white blood cells contained in said cerebrospinal liquid, the detecting comprising the following steps: + placing the bodily fluid as a droplet on the transparent substrate, + illuminating the droplet with the light source, + acquiring several successive images (I n, I n+1) of the droplet with the photodetector array, each image (I n) being formed by radiation transmitted through the illuminated droplet and including at least one elementary diffraction pattern, each elementary diffraction pattern corresponding to waves diffracted by a particle during the illumination of the droplet, + identifying, from the acquired images (I n, I n+1), mobile elementary diffraction patterns, + counting moving particles within the droplet, from the thereby identified mobile elementary diffraction patterns, and + placing the bodily fluid as a droplet on the transparent substrate, + illuminating the droplet with the light source, + acquiring several successive images (I n, I n+1) of the droplet with the photodetector array, each image (I n) being formed by radiation transmitted through the illuminated droplet and including at least one elementary diffraction pattern, each elementary diffraction pattern corresponding to waves diffracted by a particle during the illumination of the droplet, + identifying, from the acquired images (I n, I n+1), mobile elementary diffraction patterns, + counting moving particles within the droplet, from the thereby identified mobile elementary diffraction patterns, and - diagnosing meningitis if the number of detected white blood cells in the predetermined amount of cerebrospinal fluid is greater than a predetermined threshold value, by counting only the moving white blood cells.