If humans are now genetically tainted by corruption, what was perfection like? I have frequently heard the argument that various human frailties, such as dying of old age, are the result of corruption after the original sin; less frequently, but often enough to get my attention, I hear the extended lifespans in Genesis were a result of reduced corruption.

If I take this literally, though, I end up puzzled by what this perfection could have looked like.

For example, if there were no diseases or parasites before sin, then we wouldn't really have needed immune systems.  However, our immune system is impressively complex, including specialized organs that randomly shuffle the genes in immune cells until they make antibodies that accurately recognize invaders (so they can be destroyed).  However, if there were diseases and parasites but we were supposed to be perfectly immune, our immune system's design cannot do the job: all the machinery devoted to random shuffling necessarily requires time, during which we are sick.

As another example, cancer always eventually kills everyone if they escape every other malady.  There is no way to escape this fate because potassium, which is an essential element, is also occasionally radioactive; this radioactivity damages our DNA, which eventually will cause enough mutations to make cells misbehave and become cancerous.

How should we think about the initial state of perfection, given the physical barriers to it, and the sophistication of our bodies in dealing with imperfection?
Genesis 1 never calls humans or human bodies "perfect", it does near the end of the chapter call all of creation "very good." We also have no indication from scripture about increasing amounts of genetic imperfections as humanity survived though the ages, so that might infer that we are trying to read into the scripture something it never set out for or was intended to answer, namely what human perfection was like (because it never existed in anyone except Jesus, who also died) and genetics (the bible was written in times when people had no concept for it).