Because crawlspaces are not always on our minds, many times a water intrusion is not found until long after it has occurred. We rarely enter into a crawlspace which is why most water damages are not discovered immediately. When a crawlspace is damage by water and not addressed in a timely fashion bad issues can result.
One issue is mold. The presence of standing water allows for much higher than normal humidity. This high humidity will eventually affect the home's subflooring. When wood reaches a moisture content above 20%, permanent damage can begin in the form of dry rot and/or mold.
Because crawlspaces are considered confined spaces, not meant as living spaces, doing any work in them can be hazardous. Entryways are small and working areas are cramped. Professional remediation is recommended, with proper personal protection such as protective clothing, gloves, boots, and possibly respirators.