The scientific explanation is that, quite simply, your mind is playing games with you regarding the "what ifs".
Again, you are unaware of this womans HIV status, you never saw blood on her hands (this would be impossible to miss by the way) and you are assuming that there must have been blood on her fingers and it must have entered the condom.
Even if it did, getting blood on the shaft of your penis is not a risk for transmission. In order for HIV to infect someone, it must, somehow, enter the body. It would not in this scenario.
You also need to keep in mind that what is fueling your fear is guilt over the actual encounter; having a woman give you a hand job which, I'm assuming, was quite pleasurable and for which your mind must somehow believe you must be punished for by becoming HIV positive.
HIV is only a disease; one that, while terrifying for most folks, is actually the most difficult STD to transmit. God or whomever is not punishing you for having someone give you a hand job.
Your excessive fear is no different than the guilt many feel after masturbating, although in your case it appears you believe that your punishment for this event must be to get HIV.
Please stop worrying about this. When we get postings like this, we provide a realistic assessment of the RISK of what took place, setting any "punishment" retribution aside. In your case, there was no risk whatsoever.
Gail
Gail