In parallel with the introduction of modern therapeutic and pharmacological interventions that have successfully resolved many diseases and conditions, previously deemed incompatible with life, there has been a significant increase in the number of patients experiencing secondary immunodeficiency. As a result, these patients are highly susceptible to various opportunistic infections. Among these infections, pneumocystis pneumonia (PCP) stands out as one of the most frequent and potentially life-threatening ones, necessitating prompt diagnosis and treatment. Observational studies have clearly shown that PCP affects an increasing number of patients with diverse underlying diseases and varying risk profiles that lead to immune system dysfunction. The population of at-risk patients and the range of these conditions continue to expand. Surprisingly, the diagnosis is now established in populations that were not initially considered at risk, such as patients on chronic glucocorticoid therapy. This disease often remains undiagnosed and contributes to a relatively high number of fatal outcomes in patients with various primary diseases. The text summarizes the basic epidemiological factors, risk factors, presumed pathophysiology, current diagnostic options, and typical clinical course of PCP in patients living with HIV and non-HIV patients, as well as the prophylaxis and treatment of PCP. It is important to note that in most patients with severe immunodeficiency, multiple agents are involved simultaneously in causing infectious complications. Coinfection with cytomegalovirus is a very common complication of PCP. In the context of multiple infections occurring simultaneously, if a coinfection goes unrecognized and untreated, it can render the treatment of PCP seemingly ineffective. Therefore, it is crucial to pay attention to potential coinfections already during the primary diagnosis.