Smallest parasite

Pneumocystis carinii, causing pneumonia in humans, inhabits the lungs but is only 0.5–1 mm (0.02 in) long. Pneumocystis carinii is found in the alveolar spaces in the lungs and appears in two forms – the trophozoite and the cyst. The cysts are 5–7 µm and resemble a partially collapsed ping-pong ball. The organism causes pneumonia and is the primary cause of death in HIV sufferers. 60% of people have actually been infected with the organism by the age of four, but a healthy immune system prevents the disease from developing, and it is only when the immune system becomes weak that it can cause illness. The smallest parasitic animals comprise various unicellular, protozoan parasites, some of which are only a few micrometres long. Leishmania donovani and L. tropica, responsible for leishmaniasis in humans, are intracellular parasites, measuring a mere 2 mm (0.08 in) long. The varied larval stages of the malarial parasites of the genus Plasmodium are also intracellular, living within red blood cells and the liver's parenchyma cells. Toxoplasma gondii, infecting all organs in humans and responsible for toxoplasmosis, is 4-6 mm (0.16-0.24 ins) long.