Toxoplasma gondii may be the most prevalent human parasite. As many as 50% of humans worldwide, and up to 80% in urban areas, have been infected with it at some time in their lives. An estimated 60 million people in the US have active cases at any given time. It’s a single celled parasite whose favored host is cats. However it can infect and live in a host of other creatures including rats and humans.