(please read all)
It's pretty simple, both in animation and as a metaphor. Sometimes in life, however, you don't have the luxury of avoiding chliche's. Your friend's in trouble and you don't have time to create a masterpiece. The low marks are simply for those reasons which you can't help unless you give something more thought and a longer production period. Try the same thing only with a bit more nuance (get suggestions) later and work longer on it, and maybe the animation will be as good as your intentions. By the way, at the end the fade in with deep red was actually very nice.
Most of us have friends affected or killed by disease at some time in their lives, and it's hard sometimes to remember what it felt like, or know what it feels like if it hasn't happened to you. If anyone criticizes your animation, just remember that they shouldn't and cannot impugn on your compassion. I wish your friend a safe recovery, but if the worst should happen try and help those most affected by it. And if you are convinced by this to continue promoting blood donation, never stop. Join an organization and hone your talents.