Spores -
They're everywhere and will grow if the other conditions are met.
Moisture -
To sustain growth, a mold colony must have water. When the relative humidity of the air is 70% or higher, mold has enough water to grow
Condensation
Because of the high humidity level in a dirt crawl space, cool surfaces such as the earth at 55 degrees, concrete and metal surfaces will sweat with water vapor condensing out of the air.
