ref Robbins 9/e p98 Pneumocystis carinii causes bilateral diffuse pneumonitis in immunocompromised patients and no discernible disease in otherwise healthy individuals. Clinical features are to some extent age-dependent. In premature and debilitated infants, onset is subtle, staing with mild tachypnea. Within a week or so, respiratory distress is apparent, with marked tachypnea, flaring of the nasal alae, retractions, and cyanosis. The illness may last 4 to 6 weeks and has a moality rate of 25 to 50 percent. In the immunodeficient child or adult onset is abrupt, with fever, tachypnea, and respiratory distress. Deterioration progresses to death in almost all cases if no treatment is given. In both types of patient, aerial oxygen tension is low, aerial pH usually increased, and carbon dioxide retention usually does not occur