The third way moisture enters the crawl space is through the foundation wall. Concrete block foundations have a much bigger problem with this than a poured concrete foundation wall. Concrete block has two hollow cores in each block. These hollow cores have a history of filling with water as the ground get saturated during a rain, by leaving the garden hose on or by a leaking hose bib. The concrete block is very porous and will allow water to pass through it. Once in the core of the block the water will level out until it finds a compromise in a motor joint or in the block itself. At this point the water moves freely into the crawl space. If the water does not find a compromise it will slowly migrate through the block to the driest side of the foundation; either back into the dirt outside or into the air of the crawl space. Since the water came in from the outside it is not likely that it will return there unless the ground dries out extremely fast. With a poured wall foundation the weaknesses are poorly patched rod holes and foundation cracks. Both of these issues will allow water to move freely into the crawl space. If the home has a problem with a false water table the water in the crawl space can also come up through the dirt floor.
The fourth way moisture enters the crawl space is by way of small cracks that are located where the wood structure meets the foundation. This is not as serious as the other three but worth mentioning. The rim joist and sill plate should be inspected for large gaps and poor workmanship. The issues in this area are generally easy to correct.