Clinical depression is a serious mental illness that has a negative impact on how a person thinks, feels, and behaves. Clinical depression, in contrast to normal sadness, is persistent, frequently interferes with a person's ability to experience or anticipate pleasure, and significantly interferes with daily functioning. Untreated, symptoms can last for weeks, months, or years; and if not treated properly, depression can cause significant impairment, other health issues, and, in rare cases, suicide.