[Rights] do not provide an assessment of overall results [of social processes]. Instead, they determine the acceptability of actions directly. The moral equality of persons under this conception is their equal claim against each other not to be interfered with in specified ways. Each person must be treated equally in certain definite respects by each other person...Rights may be absolute or it may be permissible to override them when a significant threshold is reached in the level of harm that can be prevented by doing so. But however they are defined, they must be respected in every case where they apply. They give every person a limited veto over how others may treat him...they limit what one person may do to another...There cannot in this sense be rights to have certain things--a right to medical care, or to a decent standard of living, or even a right to life. The language of rights is sometimes used in this way, to indicate the special importance of certain human goods...A right not to be killed, for example, is not a right that everyone do what is required to insure that you are not killed. It is merely a right not to be killed and it is correlated with other people's duty not to kill you....This seems to me exactly right. Strictly speaking, talk of a "right to medical care" is dubious. Politically, or if you like tactically, I endorse such rights-talk in many contexts because it often means nothing more than "people should unconditionally be guaranteed access to medical care", a claim with which I strongly agree. But, at bottom, the idea that "there is a right to medical care" seems to me to rest on nothing (and, tactically speaking, this presents a problem in that it begs the objection "no there isn't", to which we can only reply "yes there is!" and thump the table...between such stale claims only struggle decides). But the conjunctural tactical issue is complex, because the language of human rights has also acquired such hegemony that successfully designating something a human right gives it a kind of credibility or urgency. But, of course, navigating the shoals of ultra-leftism and the reefs of opportunism is complex and difficult to do. And I think we forget at our peril that there are multiple questions here to address and multiple contexts of application (e.g. political/tactical vs. principle, etc.).
Still, it remains true at the fundamental level that the "right to medical care" rests on shaky conceptual foundations. Either we go with the natural law tradition and say that human beings have certain rights by nature and the right to medical care is one of them. But this approach is dubious and generally falls apart once we reject the religious foundations on which it was erected. Or, we can go the "political" route and designate rights as an index of what has been deemed very important and generally irrevocable by a particular political order. Of course, rights in this "political" sense are likely to reflect all of the struggles, contradictions and configurations of power that the political order rests on. So, for example, much of the content of law will reflect the demands of the system and the interests of the ruling class, but some laws will also reflect the gains of social movements.
But to say that there is a "right to medical care" in this "political" sense is not to give an argument for universal medical care provision. It is to merely state the conclusion we want to reach, namely a state of affairs in which the communal provision of medical care is written into law and socially recognized as very important and generally irrevocable. To be sure, "flying the flag" and seeing who shows up is a useful political tactic, so using the slogan "health care is a right!" has the potential to draw together people who agree with the demand so that a collective fight for it can pick up steam.
But all tactical considerations aside, it's worth keeping in mind (particularly in "academic registers" where such things matter) that the conceptual foundations of the "right to medical care" argument are shaky at best.
I know there are more sophisticated rights-views that I'm excluding here (e.g. Kantian-influenced views such as that of Habermas). But I'm still generally confused about exactly what is being said when people say that there is a "right to medical care" in some philosophically sophisticated sense. Ordinary people mean nothing more by it than "I strongly favor communal provision of health care based on human need", and I agree with this meaning, as far as it goes. But I can't say I agree with the more sophisticated defenders of that claim because I don't think I understand exactly what they're saying.