Pneumocystis carinii was originally described in 1909 by Carlos Chagas. He thought it was a cystic form of Trypanosoma cruzi. In 1910 Antonio Carini observed similar cysts in rats with experimental trypanosomiasis, but thought that it was a new, unknown organism. He sent material to his colleague Laveran for further investigation. In 1912 Laveran’s student Delanoe described similar cysts in lungs of Trypanosoma-free sewer rats. He gave the organism the name Pneumocystis carinii. The taxonomic classification is debatable, though on the basis of DNA analysis it is at present regarded as an unusual fungus. It is a microscopic organism that frequently causes inflammation of the lungs in severe immunosuppressed individuals. Disseminated infections can occur, e.g. with retinal lesions and foci in spleen and kidneys (frequently calcified). It is not easy to confirm the diagnosis via microscopic investigation, as this has a limited sensitivity. The pathogenic organism is best shown via bronchoscopy and broncho-alveolar lavage, though it can also be found in sputum induced in a non-invasive manner. The latter however requires availability of a special aerosol appliance to create a very fine spray of 3% saline solution. The technique is less sensitive. The organism can be shown with Giemsa staining and is recognizable as small, fine blue spots (the capsule is not stained with Giemsa). Gomori methenamine silver staining, which also stains the capsule, is better.