An estimated 19 million Americans suffer from depression. This sometimes crippling disease can range from mild sadness to moodiness to inability to sleep. Depression can also lead to the kind of full-blown psychosis that drove a loving husband to brutally murder his twin daughters.

"I really wasn't sure that I really wanted to do this interview," Oprah says. "A terrible, terrible thing happened. Two little girls who should be going to kindergarten, learning to read, growing up, are gone. But I agreed ... for one reason only. I wanted to understand and to help you understand how dark of a place you have to be in to kill your own children."