It may seem like a flaw that a virus could develop that would give people a compulsion to give blood and had I thought about it, I might have considered it a flaw. But, a few months ago, I saw a bit on the "Animal Planet" channel about an organism that causes ants to act like zombies for a while and then climb out on to the end of a blade of grass and hang onto to it with their mandibles until they die. The behavior enables to organism to reproduce but it is part of a cycle where it has to travel through two or three separate creatures to do this.
very weird
Interesting. But is that behavior really unrelated to anything the ants ever do naturally? Or is it just taking a regular behavior out of context? I don't know, but I'll assume it's the latter. Say, that the ants usually grab grass as a way of bringing food to the nest, and the organism screws up with their ability to figure out that the grass can't be moved before it is cut.
My problem with the story isn't with the "desire to donate blood", it's with the causality - desire to donate leads to altruism. If it was the reverse - say, that ALAS made people more altruistic (by, say, invoking social instincts normally restricted to close relationships), and also made blood feel thicker if there wasn't occasional bloodletting. Each of these on its own is possible, and the result would be more blood donation. However, that switches the causality of what was implied in the story.
If you just had people who felt uncomfortable without the occasional bloodloss, you'd end up with more blood donations, but also more self-cutting and people with leech addictions and the like. The story didn't mention these at all - and if it was the case, then it couldn't work in the way the story described it, since "altruism arises as a rationalization for blood donation" won't work if the blood donors figure out that they share symptoms with self-mutilators.
My main gripe here, though, is not "this is implausible". I don't mind implausibility. It's that I figure that the story would work just as well if the causal relationship between altruism and blood donation were reversed, so the implausibility feels, to me, rather gratuitous. Which is annoying on its own, and even moreso because it makes me suspect that there's a motivation which I'm just missing, and I'd hate that to be the case.
(Note that for all the nitpicking I still feel this is a great story.)