Feeling sad every now and then is a fundamental part of the human experience, especially during difficult or trying times. In contrast, persistent feelings of sadness, anxiety, hopelessness and disinterest in things that were once enjoyed are symptoms of depression, an illness that affects at least 20 million Americans. Depression is not something that a person can ignore or simply will away. Rather, it is a serious disorder that affects the way a person eats, sleeps, feels and thinks. The cause of depression is not known, but it can be effectively controlled with treatment.