Anyone who has ever started seedlings in the house or in a greenhouse has looked in on their seedlings at one time or another and found them laying face down on the soil. The stem is wilted at the soil line. This condition is called "damping off" and is caused by a fungus.
Several years ago I learned a trick to thwart the damping off fungus: After you get your seeds sown in the flat (of new potting mix), sprinkle a thin layer of milled peat moss over the entire surface of the flat. Also, water them from the bottom by soaking the flat in a larger container of water, rather than sprinkling from the top.
I keep an oscillating fan in the greenhouse also, which keeps air circulating when it gets really humid and "stuffy" in there.
Knock on wood, no one's been laying face down in the dirt since I've done this. With all the seed starting going on right now, I thought someone might benefit from this trick!
Happy seeding from Wild Things