<prompt> P. carini causes infection primarily in?RatsGuinea pigRabbitsHumans  <response>: Pneumocystis jirovecii causes infection Primarily in Rats. Pneumocystis jirovecii previously P. carinii is the causative organism of Pneumocystis pneumonia paicularly among immunocompromised hosts. P. jiroveci causes PCP in human with HIV if, their CD4 cells count below 200uL. 