Author's Note: Hitherto, I've ignored this paragraph because I found it too baffling even to mock. However, if it were to be taken seriously...
"He took my blood," said Harry.
"Precisely!" said Dumbledore. "He took your blood and rebuilt his living body with it! Your blood in his veins, Harry, Lily's protection inside both of you! He tethered you to life while he lives!"
Harry stared at him. "Excuse me?" he said at last. "That's sufficient? That's a worthy tether to the land of the living?" He scrunched up his face. "By 'my blood', are you including blood relatives, or..."
"No, no, your blood," Dumbledore corrected. "Which is to say, the blood he took from you."
"Wait. He's literally constructed from the components he used? That wasn't just a symbolic scaffolding for his regrowth?" Harry couldn't help it; he laughed. "Poor bastard. No wonder he's so funny in the head. He must be having the immune rejection problems from Hell - blood's made in the bone marrow, you know, and he used his father's bones. That's not even getting into Pettigrew's flesh - Poor bloke's got three different immune systems circulating inside him. Unless he's hopped up on loads of immunosuppressant potions, the Horcruxes must be the only things keeping him alive."
Dumbledore was only staring at him blankly. "Sorry - one can't hang around Hermione for years without picking up a bit," Harry added.
"I am aware of modern Muggle medicine, Harry," said Dumbledore, "but I daresay magic does not conform to ordinary physical law-"
"You're the one who said he's literally walking around in a patchwork of three different people's tissue, sir, not me. Besides which, that's just a side note - there's something much more important about what you said." Harry sat back on the bench and raised an eyebrow at the flailing thing behind the bench. Poor bloke. Truly. "My blood tethers me to the living as long as it's running through someone else's veins, thanks to my mum's protection. Correct?"
"Yes," said Dumbledore, raising a finger, "but the effect, as you are no doubt aware, would be only temporary for any ordinary transfusion: the magic of what Voldemort did is that he literally sustains himself off of your authentic blood, and-"
"Blood's made in the bone marrow. I can give anyone a permanent infusion of my blood through a bone-marrow transplant."
Dumbledore's mouth hung open. Even the miserable thing behind them suddenly went quiet and still.
"And because of Skele-Gro, I have an infinite supply of bone marrow," Harry continued. "Oh, it's going to be painful, I'll give you that, but it's much better than literally tearing your soul in two, isn't it?" He flicked a gaze up towards the ceiling. "Thanks, mum. Thanks to you, I can make an effective Horcrux out of every compatible person requiring a bone-marrow transplant in the world. And, if magic can work around compatibility, that increases the possible candidates even further." He twiddled his thumbs. "And there's always going to be more leukemia patients, so I can keep adding new anchors every year. The main problem is avoiding Muggle questions on who this one weird bloke is with infinitely many bones." He pressed his fingers together. "Suppose I should put my money into anti-graft-rejection research. After all, to Hell with seven Horcruxes - I can eventually have dozens of people walking around with my blood in their veins..."
Dumbledore had buried his face in his hands.
Author's Note: The logistics of this method of immortality were difficult for me to determine on a quick search, mainly because I'm not sure of the odds of Harry actually finding compatible recipients.
Of course, the odds would increase greatly if it turned out all members of the marrow-transplant network were subject to the same tethering to life - some folks would gladly go on massive immunosuppressant regimens from here to eternity if it meant they never died. I mean, some batty rich people in real life are considering transfusions of young blood as a way to reverse aging, so becoming outright immune to death would surely appeal to the same crowd.
I apologize for the lack of a more detailed analysis, but the mind boggles. I'm not sure what to make of the implications of bone marrow transplants being (at least in Harry's case) a valid substitute for Horcruxes.