T. gondii is a single-celled microbe around 6 µm (micrometers, 1/1,000 of a millimeter) in size, which is ~550 times smaller than an apple seed (3.3 mm)! This organism is so tiny that we can only see it using the powerful lenses of a microscope. T. gondii only survives and multiplies when it infects a living cell, which is why it is classified as a parasite. The animals that parasites infect are called hosts and parasites often have complex life cycles that involve different shapes of the parasite and multiple hosts.
Cats are one of the hosts of T. gondii, and they are the only host in which this parasite produces structures called oocysts. An oocyst is a thick-walled structure in which the parasite can survive for a long time outside a host. When cats are infected, they release the parasites' oocysts into the environment through their feces (poop). When other animals, such as birds, mice, cows, or even humans, ingest water, vegetables, or meat contaminated with oocysts, these animals can become infected.

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