Figure 1. Schematics of in vivo infection by the Toxoplasma tachyzoite and the Plasmodium sporozoite.

Left, the Toxoplasma tachyzoite. Toxoplasma infection starts by the ingestion of oocysts or tissues cysts (1) that liberate in the intestinal lumen free sporozoites or bradyzoites, respectively (2). Free parasites invade enterocytes (3), where they multiply and transform into tachyzoites, which are released in the lamina propria of the intestine (4). Tachyzoites invade the endothelium of lymphatic vessels (5), are taken up by the lymph (6), go through lymph nodes (7) and reach the blood circulation (8). Tachyzoites then cross blood endothelium barriers in immunoprivileged organs (9), for example, the brain, the retina or the placenta, or in other organs (10), for example, muscles, where they invade host cells, multiply and transform into dormant bradyzoites (11). Tachyzoites in the intestine may also cross the endothelium of blood vessels. Right, the Plasmodium sporozoite. Sporozoites are formed inside oocysts (1) in the wall of the mosquito midgut, are released in the hemocele bathed by the hemolymph (2), invade acinar cells (3) and exit in the secretory cavities (4) of the salivary glands and finally move into the secretory ducts (5). In the mammalian host, sporozoites are deposited in the dermis (6). Sporozoites can invade the endothelium of lymphatic vessels in the dermis (7) and then end up in the proximal draining lymph node (8), where they are killed. Sporozoites can remain in the dermis (9), in which case their fate is unknown. They can invade the endothelium of blood vessels in the dermis (10), reach the liver, cross the endothelium barrier of liver sinusoids (12) and invade hepatocytes (13), where they transform into merozoites, the erythrocyte-infecting form of the parasite. Sporozoites in the blood can also reach other organs than the liver (11), for example, the spleen, where the sporozoite fate is not well documented. BZ, bradyzoite; MZ, merozoite; SPZ, sporozoite; TZ, tachyzoite.