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