The organismToxoplasma gondii
The disease: Toxoplasmosis
Symptoms: Immunocompentent adults -some flu-like symptoms, possibly increased risk taking behavior, rarely schizophrenia.  Fetuses: hydrocephalus (“water on the brain”), inflammation of the retina, and later in life: blindness, deafness, mental retardation.
How it works:  T. gondii is a eukaryotic (meaning that genetically it is more related to us than to bacteria), single celled organism that appears to prefer infecting cats.  Because any one organism is guaranteed at some point to assume room temperature, T. gondii hitches a ride into the cat’s poop in a hardy spore-like form that can infect just about any mammal that ingests it, including us.  It appears to have a special liking for forming cysts in either skeletal muscle or brain.  Here is where it gets REALLY creepy. Once in the brain, T gondii can actually change that animal’s behavior (sound crazy? see here).  For instance, infected rodents are less fearful of cats.  Cat eats stupid/brave mouse and T. gondii has itself a new mobile home.  Studies have clearly shown that 15-40% of humans have been infected and that the bug does appear to change human behavior too.  Don’t believe it?  Google “Toxoplasma gondii and Schizophrenia”
Where is it in your house: Your cat, the catbox or anything that has touched your cat’s feces (water, the floor, your dog’s mouth etc) and potentially the meat in your refrigerator.  Given that so many people have clearly been infected, millions of new infections are probably happening every year, probably from infected farm animals.
What you can do to protect yourself:  Buy and learn how to correctly use a home irradiator for your food.  Short of that, cook your food well and keep pregnant mothers and babies away from cats and their end products.  Resist the temptation to rub catfood on yourself and lie down in front of your cat (but such a temptation suggests that its too late for you.)
Above all, please, wash your hands.

Toxoplasma entry at MicrobeWiki

The organism: Listeria Monocytogenes
The disease: Listeriosis
Symptoms: Fever, muscle aches, nausea, confusion, meningitis, convulsions, miscarriage of pregnancy
How it works: L. monocytogenes is a bacteria that is commonly found in soil; it is  prevalent around cattle, sheep, goats, pigs and poultry – all of which can produce a lot of Listeria in their feces without showing any signs of infection.  The main route of infection for humans is consumption of food that was processed in a facility with less then spectacular commitment to our safety.  Generally the very young, very old and those with comprimised immune systems fare worse when infected, but Listeria can cause disease in otherwise helthy adults.  The Centers for Disease Control reports 142 cases so far this year, right on track for the yearly average of about 800 cases per year.
Where is it in your house: The USDA helps to ensure that it probably isn’t.  If it were, most likely it would be in your refrigerator, growing on pre-cooked, ready to eat packaged food or on the surface of eggs. Listeria is remarkable for its ability to survive heating and to grow even in cold environments without oxygen (like inside a pack of unopened hot dogs).
What you can do to protect yourself:  Grow your own food and learn how to butcher your own meat without contaminating it.  Buy eggs with the official “USDA Grade A” (or AA) symbol only.  Not all egg producers register with the USDA, and are not subject to inspections of their facilities. Never ever water vegetables with water containing animal feces as the vegetables can harbor Listeria for a long time and make you sick.  Above all, please wash your hands.
CDC Notifiable Diseases and Mortality Tables

Listeria entry at MicrobeWiki