I am moving to a house with a finished basement. It is very damp and mildewy.
It only has three small windows and a door to the outside. There are no screens.
My son has allergies and plans on living in the basement. My first thoughts were to get screens on the windows and door for ventilation and set up a dehumidifier.
If its a leak, as previously mentioned it needs to be fixed otherwise a good scrub, a dehumidifier and a good bit of circulation (keep those windows open every now and then) and you shouldn't run into too many troubles.