Toxoplasma gondii, a ubiquitous parasite of mammals and birds in nature, is one of the most common infections of humans the world over. In the United States alone, at least 66 million persons are infected by this protozoan. Although recognized as a cause of disease in humans for more than 50 years, the probable means whereby this parasite is transmitted has only recently been elucidated.
In the life cycle of Toxoplasma, there are three forms of the parasite: the trophozoite or proliferative form, the tissue cyst, and the oocyst. The trophozoite is an obligate intracellular form that invades all types of mammalian cells and can be seen during the acute stage of the infection. The tissue cyst develops in host cells from trophozoites and may vary in size from those containing only a few organisms to those containing several thousand. Such tissue cysts may persist in any tissue and are
SWARTZBERG JE, REMINGTON JS. Transmission of Toxoplasma. Am J Dis Child. 1975;129(7):777-779. doi:10.1001/archpedi.1975.02120440005002