First major issue is circulation. The circulation in this case, typically because there aren't many pored floors in crawl spaces is the circulation can cause mold to grow faster in a tight, dark, moist environment. Simply installing a fan to pull air out of the space and outside can solve a few issues, but it can also create quite a few.

By pulling air through the space you can coax the moisture to, through capillary action, pull more moisture through the foundation. Or when the fan isn't running, you have a direct leak for dense humid air to gather in the craw space via the fan duct. There are systems out there that you can research to find solutions to circulation. However, there is a more permanent solution to crawl spaces' moisture issues.

Encapsulation is a process that takes a high density vapor barrier (one much less permeable than a Vapor Diffuser which most contractors use on the above grade house construction.) and actually wrapping the walls and the floor in it. Essentially locking the moisture under the barrier where it can then be directed to a Sump Basin and be ejected from the house using a sump pump and discharge line.

Encapsulation, depending on your desired outcome, can be designed as either a moisture lock out or a functional storage space. By adding sub flooring under the high density Vapor Barrier you're ensuring moisture flow to the sump as well as creating a more stable floor in which to cause less friction with the above Vapor Barrier. If it's going to be a high traffic crawl space, with much moving and storage on a regular basis, then a second sheet of Vapor Barrier can be installed on top of the encapsulation to add extra strength. If someone wanted to add yet more sub flooring on top of the vapor barrier, which could also be an option.