Hypothetical situation, only the answer is important, no 'Oh, I'd never do heroin in the first place' or 'I'd never share a needle if I did herion'....just answer the ultimate question. OK?
You and your best friend since birth are together, in a hotel room, alone.
You're about to shoot up and your needle breaks. You have no back-up and no way to get a new one and you're dope sick.
Your friend offers to share his/her needle with you and swears on their life that they're disease free.
You accept, and believe the friend because you have known them for life and trust them. Plus, you're a heroin addict and you're sick so you are motivated by that.
6 months later you find out he/she gave you HIV when you shared that needle.
Whose fault is it? Is it your fault for agreeing to share the needle and believing her/him? Or should your friend take the blame for not disclosing or not knowing for sure of their HIV status?
You and your best friend since birth are together, in a hotel room, alone.
You're about to shoot up and your needle breaks. You have no back-up and no way to get a new one and you're dope sick.
Your friend offers to share his/her needle with you and swears on their life that they're disease free.
You accept, and believe the friend because you have known them for life and trust them. Plus, you're a heroin addict and you're sick so you are motivated by that.
6 months later you find out he/she gave you HIV when you shared that needle.
Whose fault is it? Is it your fault for agreeing to share the needle and believing her/him? Or should your friend take the blame for not disclosing or not knowing for sure of their HIV status?

Ultimately, I think people should take responsibility for their own actions, and I'm going to say it's your fault.
It is a good question, though. I wonder if there are any cases that made court.
Anyway at the risk of putting my foot in my mouth and being so sure that it's not a crime or something you could sue over (though I AM pretty sure...in the US anyway), I'll just link a wiki page I came across....Still, reading it, it mostly talks about sexual transmission. It does mention throwing or spreading of body fluids but not specifically if IV drug users are protected, it only mentions truly innocent parties like prison guards (inmates like to throw body fluids on them...a lot, so it makes sense to make that a serious crime).
Anyway, here's that:
<a href="http://en.wikipedia.org/wiki/Criminal_transmission_of_HIV" rel="nofollow">http://en.wikipedia.org/wiki/Criminal_transmiss...</a>
I don't think I answered your question.
Anyway:
On things like that, NEVER trust someone else. If i'd later find out she KNEW and lied, i'd literally kill her in return.
It may be useless and even detrimental to place blame and hold a grudge, and of course I believe in taking personal responsibility for whatever you were personally responsible for (in this case, making a stupid decision), but sometimes something really is somebody's fault.
I think the friend made the bigger mistake here.
Fucking junkie piece of shit.
If, on the other hand, the friend honestly didn't know when he told me he was clean, that would be a different story. I would not blame him if that were the case.
Final decision: neither. But you shouldn't have been doing heroine in the first place :P
Haha just kidding, it would be my own damn fault. As soon as I decided to start using heroin I immediately accept the risks of heroin usage, one of which being AIDS. Then, as soon as I decided to stick a needle into my arm that I don't know for a fact is clean, I again accept the responsibilities of using a possibally dirty needle, again one of which being AIDS.
There is a line of decisions that I would have made, tracing directly back to the first heroin usage, every step of the way pointing to my own fault.