As we all know, wounds are healed by repairing tissues and creating more cells to replace the damaged ones. We also know that cancer is uncontrollable replication of cells, made worse by the fact these new cells have incomplete/messed up DNA. Here’s the idea:in the Old Guard world, there is some sort of substance/gene that causes a similar uncontrollable replication of cells, though without the messed up DNA, and the “immortals” are people with a specific gene that counteracts this. Their genes mean their bodies will kill the extra cells when there is no need, and allow the growth when there is (i.e. when they are wounded). This is actually pretty normal for people, as our bodies usually have cancer cells, but kill them before they become a problem. The immortals are only special because their bodies are much better at ensuring their DNA doesn’t get corrupted. This would also explain why they don’t age, as aging is caused by natural degradation of DNA over time (our cells are replicated millions of times, and any mistakes made are going to continue to get worse the longer they are passed down). Thus when the immortals lose their “immortality,” it’s actually just their DNA finally losing that special trait that kept the replications in perfect order. Another thing that gives credence to this theory is Booker’s family. I’m pretty sure his kids died of cancer, while he himself got immortality, which sounds to me like they didn’t get that special gene which would have allowed them to control the replications. As for why they dream of each other before they manage to meet (crack warning ahead), aliens. Obviously the ones releasing the cancer substance are extraterrestrials who wanted to experiment with it. Whatever it is, it likely only affects carbon-based life forms, and the aliens probably wanted to test why some creatures could control the replications and others got cancer. To encourage the ones with unusual reactions to the substance to get together for easier study, the aliens would obviously use their telepathy powers to pester them into finding each other.
