Mycoplasma pneumoniae is a wall-less bacterium that causes interstitial pneumonia in young adults. Elevated cold agglutinins (a classic clue) are found in about half of the patients. The cold agglutinins are IgM antibodies. M. pneumoniae may be diagnosed by sputum or complement fixation. Haemophilus causes bronchopneumonia in babies and children, and may occur in debilitated adults. Klebsiella causes a bronchopneumonia with patchy infiltrates involving one or more lobes. Think red currant jelly sputum. It frequently occurs in debilitated patients, diabetics, and alcoholics. Note that these organisms are highly encapsulated and produce mucoid colonies on lab media. Legionella lives in contaminated water sources such as air conditioning systems. Ref: Ray C.G., Ryan K.J. (2010). Chapter 38. Mycoplasma and Ureaplasma. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.