In response to T. gondii, the body produces natural killer cells and cytotoxic T cells. These cell types wage war against cancer cells. Cancer can shut down the body’s defensive mechanisms, but introducing T. gondii into a tumor environment can jump start the immune system.

“The biology of this organism is inherently different from other microbe-based immunotherapeutic strategies that typically just tickle immune cells from the outside,” said Barbara Fox, senior research associate of Microbiology and Immunology. “By gaining preferential access to the inside of powerful innate immune cell types, our mutated strain of T. gondii reprograms the natural power of the immune system to clear tumor cells and cancer.”

How they keep it from being infectious:

[T]hey delete a Toxoplasma gene needed to make a building block of its genome and create a mutant parasite that can be grown in the laboratory but is unable to reproduce in animals or people.

Vaccine used in live mice with cancer:

Published laboratory studies from the Geisel School of Medicine at Dartmouth labs have tested the cps vaccine in extremely aggressive lethal mouse models of melanoma or ovarian cancer and found unprecedented high rates of cancer survival.

… and since I mentioned toxoplasmosis affects human behavior …

Heres a link talking about how toxoplasmosis can affect human personality.