Pneumocystis jirovecii pneumonia has historically been one of the most common opportunistic pneumonias and life-threatening infectious complications in HIV-infected patients. After the introduction of combination antiretroviral therapy, the incidence of Pneumocystis pneumonia and other opportunistic infections has decreased dramatically. Nowadays Pneumocystis pneumonia still occurs in patients unaware of their HIV status, in those not receiving combination antiretroviral therapy, or in those in whom it is ineffective due to resistance. Age factor is the diagnosis delaying one: patients aged more than 50 years are diagnosed with AIDS later than younger persons. Pneumocystis was thought to be a species of protozoa. Over the last 20 years, Pneumocystis has been shown to be a fungus, to be genetically diverse, host species specific, to colonize individuals with minor immunosuppression, and to cause clinical disease by "new" infection in addition to reactivation of latent childhood-acquired infection. Recently, the microorganism Pneumocystis carinii causing disease in humans has been renamed to Pneumocystis jirovecii. This article presents a clinical case of late diagnosis of Pneumocystis jirovecii pneumonia in a 62-year-old patient unaware of her HIV status and a review of literature reflecting epidemiological issues of Pneumocystis jirovecii and latest discoveries related to Pneumocystis as well as the rationale for renaming it.