I kept thinking how funny it was that the people and the system he so hated were actually trying to help him, though if only to protect themselves. Here is a case where it's vital to care for a fellow human being ... but only because they have to be cured of something bad for you. It's a selfish empathy is the only way I can describe it.
Ultimately we're dealing with a story of being trapped by death which is inevitable for us all. Marciano may be dying quicker than the rest of us, but it's all the same, isn't it? Yet one layer down we're also dealing with a story of an immigrant - and here we get parallels with Kafka's The Metamorphosis - who is seen as a pest in a society that wants to "clean" him. He's not just sick with tuberculous, but with being Mexican (to be absolutely blunt).
I think it's important to also consider that this story is told only from the point of view of Marciano. We never get any hint what other characters are thinking, though we can infer their motives (the bartender probably talked to someone about the guy coughing in his bar), and so we feel trapped along with Marciano, a man who doesn't really speak the language, who lives on the edges of a beautiful and clean California, and so sees most people around him as out to get him. He even thinks about paranoia later in the story, so we feel how alone he is.
Now what's funny is that he isn't alone, there are a whole lot of very professional people who are very interested in him, but only because they want (need) to cure him. Once he's cured they'll be done with him, but for now he's the most popular lonely guy in California.
And there is a slight religious parallel here with the idea of being cured equaling being saved, where he has to do serious penance to be "better" or at least be converted well enough to be tolerated by the conquerors. I even thought of Moses from the line: "It was like climbing a mountain backward - no matter how many steps you took you never got to see the peak." There's sort of a Moses image here, the mountain where he takes his commandments (his medicine from the godlike doctor), but Moses got to see Jerusalem and he doesn't think he will see a promised land.
And considering how terrifying the death image is at the end I think he knew quite well there was no promised land. He was literally a throwaway human drinking not from the fountain of life, but from a busted sprinkler people use to care more for the grass than for the people walking on it.
Something else to consider - and this would address any possible racism (towards either Mexican's or white people) is that Marciano is in the wrong and he's literally and actually doing a bad thing. He really is sick and he needs to be cured. Had Marciano been given some non-specific ailment then we could blame the author for having an agenda and making the Mexicans the "good guys" and the whites the "bad guys". But it's much more complicated here. Marciano rightly feels trapped, but he's wrong. People want to help him, but they're only doing it to save themselves - which they weren't able to do when he spits on them.
And since we don't get any real characterization here, his being so in the wrong makes up for a lack of moral exploration. We can only deal with the situation and read into it what we will. And what we see is complicated: the Mexican laborer is uneducated and paranoid of the white people trying to help him, the white people are not really caring people, they just don't want to die. Yet he is being helped and they are helping him, so is the fact that the health workers are only being selfish a bad thing? Does a good deed done for a selfish reason cease to be a good deed?
What I also feel is important here is that trapped feeling that so many people feel. I currently work with an entire team of Mexican and South American laborers who do housekeeping for the company I work for - I am their in-house supervisor, not their boss (hefe) - and all of them have told me stories of how they are taken advantage of and there is nothing they can do.
For example I have to grade their work everyday on a very strict scale and if that scale falls below a certain threshold for whatever reason they get a bad mark. Now I have to grade their work as it is even if I know full well there were circumstances beyond anyone's control that led to the bad mark. And when I grade them bad they get in trouble. So when they are only given half the amount of people needed to do a job they get marked bad because the work wasn't done and they get blamed and then fired. Meanwhile the hefe (their actual employer) keeps the money that could have been used to pay more workers and he knows his contract with us guarantees his income. These people are being literally used.
And so what am I supposed to do? Grade them good even though it's bad work? Then I lose my job. And am I going to sacrifice my income in a battle I can't win? Yet I know they get mad at me when I grade them bad and they see me as part of the system they know is unfair.
We're not allowed to even be human anymore - we're all treated like animals, and we treat each other like animals, and we just look out for ourselves, and it's what is wrong with the world. It's a terrible, unfair system and I hate it. And it really exists. And it's killing people.