a fungus which is generally found in the lungs of healthy people, without causing disease until the person's immune system becomes weakened. Risk factors Pneumocystosis occurs predominantly in people with HIV/AIDS. Other risk factors include chronic lung disease, cancer, autoimmune diseases, organ transplant, or taking corticosteroids. Diagnosis Diagnosis of Pneumocystis pneumonia is by identifying the organism from a sample of sputum, fluid from affected lungs or a biopsy. A chest X-ray of affected lungs show widespread shadowing in both lungs, with a "bat-wing" pattern and ground glass appearance. Giemsa or silver stains can be used to identify the organism, as well as direct immunofluorescence of infected cells. Diagnosis in the eye involves fundoscopy. A biopsy of the retina and choroid layer may be performed. In affected liver, biopsy shows focal areas of necrosis and sinusoidal widening. H&E staining show extracellular frothy pink material. Typical cysts with a solid dark dot can be seen using a Grocott silver stain. Differential diagnosis Pneumocystosis may appear similar to pulmonary embolism or adult respiratory distress syndrome. Other infections can present similarly such as tuberculosis, Legionella, and severe flu. Prevention There is no vaccine that prevents pneumocystosis. Trimethoprim/sulfamethoxazole (co-trimoxazole) might be prescribed for people at high risk. Treatment Treatment is usually with co-trimoxazole. Other options include pentamidine, dapsone and atovaquone. Outcomes It is fatal in 10-20% of people with HIV/AIDS. Pneumocystosis in people without HIV/AIDS is frequently diagnosed late and the death rate is therefore higher; 30-50%. Epidemiology The exact number of people in the world affected is not known. Pneumocystosis affects lungs in around 97% of cases and is often fatal without treatment. History The first cases of pneumocystosis affecting lungs were described in premature infants in Europe following the Second World War. It was then known as plasma cellular interstitial pneumonitis of the newborn. Pneumocystis jirovecii (previously called Pneumocystis carinii) is named for Otto Jírovec, who first described it in 1952. References Parasitic infestations, stings, and bites of the skin Protozoal diseases Fungal diseases