Toxoplasma gondii is a single celled parasite that lives in a cat’s intestine. While it prefers felines, it can live in humans and other animals. In fact, about 1/3 of humans are hosts to it. Normally this isn’t a problem, but it is for people with suppressed immune systems. The interesting thing is that a human body’s reaction to T. gondii is similar to its response to cancer tumors.
This leads to the idea that perhaps this parasite can be used as a cancer therapy. T. gondii stimulates the body’s immune system to fight cancer. While cancer can shut down the immune system, T. gondii stimulates it. Scientists have created a version of the parasite that can be grown in a lab, but can’t grow in animals/people. This may lead to an effective cancer drug that helps the body fight the disease.