[8] Also, a condition similar to acute respiratory distress syndrome (ARDS) may occur in patients with severe Pneumocystis pneumonia, and such individuals may require intubation.

[10] The disease attacks the interstitial, fibrous tissue of the lungs, with marked thickening of the alveolar septa and alveoli, leading to significant hypoxia, which can be fatal if not treated aggressively.

Oxygen is less able to diffuse into the blood, leading to hypoxia, which along with high arterial carbon dioxide (CO2) levels, stimulates hyperventilatory effort, thereby causing dyspnea (breathlessness).

[citation needed] In addition, in symptomatic cases of P. jirovecii pneumonia, the overgrowth of the fungus is associated to a co-infection with trichomonads, unicellular flagellated parabasalid protist (Parabasalia) of the family Trichomonadidae.

[12] The diagnosis can be confirmed by the characteristic appearance of the chest X-ray and an arterial oxygen level (PaO2) that is strikingly lower than would be expected from symptoms.

[citation needed] The diagnosis can be definitively confirmed by histological identification of the causative organism in sputum or bronchoalveolar lavage (lung rinse).

[citation needed] Pneumocystis infection can also be diagnosed by immunofluorescent or histochemical staining of the specimen, and more recently by molecular analysis of polymerase chain reaction products comparing DNA samples.

[citation needed] In immunocompromised people, prophylaxis with co-trimoxazole (trimethoprim/sulfamethoxazole),[15] atovaquone, or regular pentamidine inhalations may help prevent PCP.

Other medications that are used, alone or in combination, include pentamidine, trimetrexate, dapsone, atovaquone, primaquine, pafuramidine maleate (under investigation), and clindamycin.

[17] Infections with Pneumocystis pneumonia are also common in infants with hyper IgM syndrome, an X-linked or autosomal recessive trait.

The possibility of person-to-person transmission has recently gained credence, with supporting evidence coming from many different genotyping studies of P. jirovecii isolates from human lung tissue.

An unusual rise in the number of PCP cases in North America, noticed when physicians began requesting large quantities of the rarely used antibiotic pentamidine, was the first clue to the existence of AIDS in the early 1980s.

Much of the incidence of PCP has been reduced by instituting a standard practice of using oral co-trimoxazole (Bactrim / Septra) to prevent the disease in people with CD4 counts less than 200/μL.