I think AV's theory has some merit, although I too struggle with how far to take it.
Like human beings, each animal is unique within its species, with its own unique compliment of genetic predispositions and gene expression, and its own unique 'ideal' diet. The diet it actually has access to will deviate from this ideal to varying degrees, both in general due to where it lives and also temporally due to natural 'lack and larder' (terminology from Jon Young and Tom Brown Jr.) cycles imposed by the seasonal availability of different foods. This deviation means that no animal, even in the 'wild', actually eats its ideal diet, although it might come close, to varying degrees and at varying times of the year. So the fact that wild animals (and wild humans) do sometimes suffer from disease and parasite infestations shouldn't surprise us.
Those animals (and wild humans) that choose to live in areas that force their diet to deviate from their genetic ideal will tend to show more frequent and more severe signs of disease, and will be more susceptible to parasite infestation and bacterial/viral infection. Those animals that choose to live in areas that allow their actual diet to closely match their genetic ideal will show fewer and more mild signs of disease, and will be less susceptible to parasite infestation and bacterial/viral infection.
On the other hand though, parasites, bacteria and viruses all benefit from not killing their hosts, so if a parasite/bacteria/virus starts using human beings as a host one would expect that while those initial infestations might result in serious symptoms in the host or even death, evolution would push the parasite/bacteria/virus towards a more symbiotic relationship with its host so as to avoid ending its genetic lineage when it kills its host.
That's my current thinking on the topic anyway. I look forward to reading what others share.
