Pneumocystis jiroveci (P. jiroveci) is the etiological agent of pneumocystis pneumonia (PCP) in immunodeficient patients. The increased interest of clinicians in this particular pathogen during the past decade was prompted by rising numbers of patients with immunosuppression caused by AIDS, chemotherapy, or organ transplantation. Premature, seriously ill infants at intensive care units constitute a potential risk group for infection with P. jiroveci. Recent advances in medical sciences, owing mainly to developments in molecular biology, permitted the verification of the taxonomic position of pathogens and contributed to a better understanding of new aspects of pathophysiology and pathogenesis of PCP. It has been demonstrated that the genus Pneumocystis represents a heterogeneous group of opportunistic fungi exhibiting narrow host specificity. Pneumocystis jiroveci is the species which is specific for humans. The present paper outlines the clinical symptoms of PCP in infants, currently used diagnostic methods, and treatment procedures in PCP.