The clinical profile of fever, cough and X-ray revealing bilateral pneumonitis and presence of diarrhea and confusion indicates the presence of Atypical pneumonia. The organism causing atypical pneumonia out of the given choices is Legionella pneumophila Confusion can be explained by hyponatremia due to SIADH. The clinical manifestations of Legionnaires' disease are usually more severe than those of most "atypical" pneumonias. Clinical Clues Suggestive of Legionnaires' Disease Diarrhea High fever (>40degC; >104degF) Numerous neutrophils but no organisms revealed by Gram's staining of respiratory secretions Hyponatremia (serum sodium level <131 mg/dL) Failure to respond to b-lactam drugs (penicillins or cephalosporins) and aminoglycoside antibiotics Occurrence of illness in an area where potable water supply is known to be contaminated with Legionella Onset of symptoms within 10 days after discharge from the hospital (hospital-acquired legionellosis manifesting after discharge or transfer)