As the previous answer says, the foul odor is a result of the dampness in the crawl space.
High humidity levels favor mold growth and rotting of every organic surface in the crawl space, and that includes your floor joists, and sub floor.
The air from the crawl space is naturally sucked in by a a physics principle we call the "stack effect".
As the warm air rises and exits the house through the openings in the above floor, a new air supply is sucked in from the lowest levels of the building, meaning the basements and the crawl spaces. 1/3 of the air you breathe in your home right now is coming from the crawl space, naturally.
The problem is that, since your crawl space is damp and moldy, you are breathing those spores and all that filth as well. Rather than thinking about filtering the crawl space air, I'd suggest you think about completely eliminating the moisture and the mold down there through a relatively new process called crawl space encapsulation.
An independent research organization called Advanced energy along with other reputable institutions such as Habitat for Humanity, conducted a ton of studies on crawl spaces and crawl space remediation systems, and the studies are all conclusive: the best way to treat a crawl space is to seal it completely, from the outside air and isolating it from the ground, making it a part of the internal envelope of the house.
This is accomplished by lining the whole crawl space with a thick 20mil liner, sturdy like a pool liner, and then sealing it air tight. Then providing either a dehumidifier or a crawl space conditioning system to deal with eventual condensation.
What happens is, your crawl space stays dry and mold free, thus no foul odors. Better indoor air quality. You also discourage moisture loving pests and wildlife from moving in.
But the biggest benefits of crawl space encapsulation are in terms of energy efficiency. In average 18% savings in energy bills, because humid air costs more money to heat and cool. If you have ducts in the crawl space, the ENERGY loss without the encapsulation can be as hing as 50%, meaning that half of what you pay to coll your home is being wasted.
Encapsulation is a relatively inexpensive improvement that really pays for itself in energy savings along.
I am including several links below on crawl space science, including the one to the Advanced Energy/Habitat for Humanity and a step by step guide to the process from the Energy Boomer.
