I just watched the Halo 4 terminals and one of them call my attention, I think the name is "Charum Hakkor" where the Lord of Admirals attacks a Forerunner planet because there was a flood infestation, the terminal description is Humanity true purpose, or something like that. Later in another terminal video there is the Librarian talking about discovering that actually the humans where attacking and fighting the Flood. Why did the humans never communicate with the Forerunners regarding the Flood menace?
Well, I suggest you that you need to check the "human-forerunner war" in Halopedia. To be short, the name you mention above is the capital planet of the human empire, which was captured after a 53-year long battle between the warrior-servant led by the Didact and the human forces. The reason why they didn't communicate was clear, in my opinion. Because from the very beginning, when the floods invaded, they expanded for survival and thus resulting in the total conflict between human and forerunner. If they were looking for help, they would have to accept the forerunner as their savior and ally, and eventually lose control with the power, which could lead to their overall destruction.
The forerunner saga books explained that while the humans were at war with the forerunners we were both also fighting the flood on second fronts. We were losing the war to the forerunners but we're beating the flood, while the forerunners were beating us but losing to the flood. Pretty sure that's why we were given the Mantle which pissed off the Didact because he saw us as being inferior and not worthy of it. So ya not much communicating of any kind going on at that point.