An apparatus for detecting, identifying and measuring infectious disease causing microorganisms is described. The apparatus utilizes a plural medium arrangement in which a test liquid containing a microorganism is simultaneously introduced. Different media are utilized for the plural medium arrangement and at least one of the media can contain a respiration inhibiting material and/or other reagents such as antibiotics. Dissolved oxygen measurements of the media are taken as a measurement of microorganism respiration. Oxygen electrode signals within different media are compared and differential respiration variation rates are determined and identification signals produced. The identification signal resulting from the multiple respiration rate determinations is compared with known values to identify and measure the microorganism. The drug sensitivity of a microorganism is also judged when at least one of the media contains a drug for which the sensitivity of microorganism is to be determined.