I think it should read, "Depression is a horrid disease". And yes it is a disease, it's lack of a hormone or more to the brain, like diabetes is a lack of insulin. Only trouble is you can't have homones injected into your brain, so they have to be made in the body, either with antidepressants or weekly therapy.
Depression may be "reactive" (a normal response to unpleasant experiences or difficult circumstances) in which case it is known as a "neurosis" and it might not be considered a disease (I think that is what previous posters had in mind). However it can also arise endogenously (without apparent environmental causes) and be associated with delusions and other symptoms, in which case it would be called a "psychosis" and conventionally regarded as a "disease" (one of the major schizo-affective conditions. I think Duolingo's use of the word "disease" is OK in the sense that it is consistent with medical orthodoxy.