What was the intention of the designer of diseases and parasites and carnivoires?
In the context of my "conversation" on this thread, I think you mean motivation rather than intention; kind of a second order intention. Strictly speaking disease could be a design with no underling reason, it need merely be intended and brought about.
I don't know if those things are a design, much less motivation there might be, but I can relay a speculation I've heard: human frailty is an opportunity to show our quality (or not). Kind of a supernatural version of tough love. Obviously many other reasons could be propounded, for example they may not be intended at all but somehow be the natural result of "the fall."