Valis,
Let us imagine a primitive society, existing at subsistence level, and with no physical infrastructure to speak of. Their environment is simply nature. They are hunter-gatherers.
Let us suppose somebody is born into this society blind. Will he be "disabled" (in the sense of being unable to perform certain activities that people in his people group generically can?) Yes. He will not be able to run through the woods. He will not be able to effectively hunt many types of prey; he will be more at risk for death by predation, unless protected.
None of these things is imposed on him by society in any way. They are imposed by nature. They have nothing to do with how his society is set up, and they most certainly are not "inflicted on him" by society. (Depending on its ethics and social structure, the society may or may not try to alleviate the impacts of these things on him).
So there is nothing fundamentally social or infrastructure-induced about disability. Disabled people already have problems getting around, for example, before there is any infrastructure at all -- on the bare brown earth, they still have more trouble getting around. (Depending, obviously, on the disability, but I'm using ora's example now).
Having grasped this simple point that disability is not *inherently* a socially imposed condition, it is fairly trivial to grasp the further point that many or most of the effects of disability in the modern world are likewise natural, and not socially imposed. Society does not "impair you by preventing you from getting around," for example; there is a natural difficulty in getting around. Society may accomodate it to varying degrees.
Even let's take your example, valis, of snow on sidewalks. Undoubtedly it's true that too much snow on sidewalks is terrible for the disabled; but too much snow or ice at all is. Civilization may fail sometimes to ameliorate that as much as would be possible, and it may sometimes aggravate it; it does not create it.
Your other examples, and some of ora's, highlight once again the fact that society can *fail to ameliorate* a disability, leaving it in as bad a state as it would naturally have been; and it can even relieve some hardships for most people, but not for those with the disability. None of these constitutes disability being socially defined.
" Especially with less severe disabilities than full blindness or full inability to walk, the severity or even existence of disability can be drastically different in different societies, cities, classes."
This, of course -- minus the word "existence" -- is true. What is false is the claim that society *creates* disabilities. It is not only false, but quite silly.