It all has to do with the way your house was built. There could have been a problem from day one. It is a common building practice to construct homes with crawl space vents. Building code has called for crawl spaces to have open vents for air drying purposes, but this is an outdated practice that is harmful to the house and its occupants.
Some resources still encourage open crawl space vents, but the hard truth is that this is what causes a lot of problems under and throughout your home. Open vents are invitations that allow outside air, water and pests inside. Why would you want those things inside your house?
More than 50% of the air you breathe in your house comes from the crawl space. That means that whatever is in your crawl space also is in the rest of your home and affecting you and your family. This is because air is drawn up through the house in a chimney-like pattern from bottom to top. This also is what is known as the stack effect.