What does a decaying human body smell like?

EEEEeeeeew it definitely sounds like something dead grosssss, I know from watching tv when the detective/cops look at a body that has been dead for awhile they use that stuff under their nose cause of the smell, I know a cop that saw a dead body that was in a house for like a week and he said he threw up soon as they went in, he said the smell stuck in his uniform even after they left, he couldn't even describe it.........