To prevent mold, mildew, etc. you needto prevent moisture from getting into your crawlspace.
The first area to address is ground moisture. Be sure that there is not water leaking in through the foundation. verify that the foundation is relatively water tight & rain water is directed away from the building. Also put a vapor barrier (Like poly sheeting) over as much of the ground as you can.
The second area to manage is airborn moisture. The common solution has been to vent a crawlspace to the outside air, reasoning that the outside air will dry the crawlspace. The fact is, however, that under most circumstances the reverse is true. This is because the crawlspace is cool, it's temperature being governed by ground temperature & the outside air is warmer & more humid. The result then being that once the hot humid air enters the cooler crawlspace, rather than picking up moisture, it cools, causing condensation, incrasing the dampness. The best solution is always to prevent the moisture from entering the crawlspace. Don't add vents & whatever vents are there, seal shut. Depending on your climate, you may also consider insulating the perimeter foundation walls, perhaps with styrofoam. If it's not a straight foundation wall, you may also consider something like spray-on foam.
Once you have isolated the crawlspace from the outside enviroments, you may need to include the crawlspace in your house's air management system (Heat & AC). In your case be careful. Because of the health problems already experienced, be sure that you are completely seperated from the damp ground.