Cats are the source of the parasite Toxoplasma gondii which infects many people but only causes disease in an unlucky few. In this article in Microbiology Today (pdf) Fiona Henriquez and Craig Roberts describe the history of research into this important organism and the direction of future investigations which hopefully will lead to the discovery of a treatment for the infection:
A century of Toxoplasma gondii research is a protozoan parasite that can be transmitted directly from cats to humans through faecal contamination of food, or indirectly from cats to livestock and then to humans through undercooked meat. Around 30% of humans in the United Kingdom are infected, and as such, harbour dormant cysts in their brain, but few have overt symptoms of disease. Neurological disease can occur in these people if they become immunosuppressed. The possibility that apparently healthy people with infection are more likely to develop psychiatric disease, including schizophrenia and depression, is under investigation. Infection during pregnancy can cause abortion or foetal infection. Congenital disease can result in systemic, neurological and progressive eye disease. No vaccine exists for prevention of infection or disease and current drug treatments are not entirely effective.