How serious is depression?

Can be very serious. Depression can be very debilitating and seriously impair a person. If associated with thoughts of suicide can even be life threatening. If someone has symptoms of depression they should discuss with their doctor.
Varies. Depression is exceedingly common. It varies from minor to being extremely debilitating, or even fatal. Please see your doctor for proper screening and referral if necessary.
Depends. Like any illness, depression exists on a spectrum. Think of the progression of cold to bronchitis to severe pneumonia. Depression is no different- may be mild and not need medication, to life threatening.

Related Questions

I'm wondering how serious is clinical depression? Why is it called that?

Depression can be. Mild, moderate or severe. I think that the term clinical depression is for many, a good way to distinguish between a disorder that benefits from treatment ; depressed mood or depression that is mild enough not to impair function. Read more...