By reading this chapter, readers will be able to understand what is pneumocystis carnii pneumonia (PCP) infection and how it affects immunocompromised patients and on prior treatment different diagnostic parameters are performed for sampling and afterwards treatment is recommended by a physican. Readers will be able to distinguish PCP patients infected with HIV/AIDS and PCP patients infected without HIV/AIDS. Also, by going through this chapter, readers will be able to distinguish between pneumonia, PCP fungal infection and the compatibility of COVID-19 with PCP fungal infection. Different treatment strategies are discussed in this chapter in order to treat PCP infected patients and risk factors are also considered so that by keeping in view these risk factors preventive measures are adopted to treat PCP fungal infected patients. Epidemiology of PCP infection is elaborated so that reader will be able to understand whether it is endemic or pandemic infectious disease. In addition, the pathophysiology of PCP is discussed so that reader should come to know its mode of transmission in lungs where this fungal infection starts replicating. Importantly, different case studies are discussed in this chapter for the purpose of understanding that more than one infection also affects PCP infected patients and different antibiotics therapy are performed along with adjunvtive corticosteroids therapy for better recovey of the patient. It is the expected responsibility of readers after studying this chapter to provide awareness of PCP infection among health care professionals and people.