Back in the 50's North Carolina adopted a building code from Minnesota. Not sure why, but they did. One of the requirements of the code made it necessary to have the areas that we all know as crawl spaces be ventilated. Come to find out, this code has probably caused countless numbers of health issues with the people living over these spaces.
Here in NC, when the summers are hot with temperatures into the 90's and humidity also in the 90's, one could not ask for a better place to grow mold than in the average crawl space. Not long after the hot, moist air enters the cool crawl space, the water is pulled out of the air and the environment becomes ripe for mold growth, and insect infestation. The air in a house flows upward and it has been estimated that 20 percent of the air in the main level over a crawl space was recently in the crawl space.
To combat this problem, we are sealing crawl spaces. A thick membrane is installed over the ground so that all moisture gets blocked from below. Around the perimeter of the crawl space, a rigid insulation board is applied and all vents are sealed up. The crawl space then becomes what is referred to as "Sealed". For more detail on this technology I would suggest that you go to crawlspaces.org This site is the result of an intensive research study concerning crawl spaces in our Southeastern climate.
A vented crawlspace in the Southeast may very well be the worst code ever created. I, for sure, will never specify another one.