What can I do about my unlivable housing conditions?

I have been renting a home in missouri for about 9 months now and my landlord is beginning to become a problem. When we moved in there was one room and bathroom that were to be his "fall project" last year. needless to say he hasn't attempted to fix either room. the spare bedroom has mold and mushrooms on the ceiling and floor as a result of the roof leaking (which was abother fall project) We don't have have a septic tank so all of our sewage runs off into the field next to us. I don't know if that deems my home unlivable but I just don't know what my options are anymore, we never signed a lease it was a verbal month to month agreement so the only thing I have in writing are rent receipts. someone help!! what can I do??