Most cat people are familiar with Toxoplasmosis. It's the most often cited villain by the proponents of the American Bird Conservancy's Cats Indoors! program & doctors as to why cats are bad - bad - bad. Cats are cited as being responsible for spreading this dread disease to unsuspecting humans. The cat haters, of course, ignore the information provided by the Centers for Disease Control that most people who contract Toxoplasmosis get it from undercooked meat (mostly pork).
This study (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0023277) shows that rats who are infected with Toxoplasmosis (a brain parasite) lose their fear of cats, one of their natural predators, and are actually attracted to them. This allows the parasite to complete its life cycle in a cat's intestine.
Don't get me wrong. I believe that cats should be kept indoors.
There are too many dangers on the street. There are people who take pleasure in hurting cats, so keeping cats out their reach is necessary. But, feral cats exist because of irresponsible humans and they have been and will always exist. All we can do is continue to try to reduce their numbers through Trap/Neuter/Return and education. Feral cats are the victims in this scenario, not the villains. And they don't deserve to be killed in shelters simply because they don't have an "owner."