I guess it's kind of like Legionaries disease. It's actually a fungus growth that is promoted by moisture build up on your evaporator coil and inside the air ducts under the dash. For whatever reason this is not very efficient. There are some pretty fancy cures out there. One, offered as a warranty repair by GM, is the installation of a timed relay.
Air continues to circulate through the air duct and evap.
That way the car doesn't sit in the garage overnight with damp components. If you don't want to go to all that trouble, you can reduce or eliminate the odor by using a disinfectant. Of course your car now smells like the Airport just after the cleaning crew went through. This is typically at the rear edge of the hood underneath the windshield wipers on the passenger side. This process may have to be done a couple times before you get rid of the odor.