This is a type of "life" somewhere below even a virus, though with the same self-replicating and DNA-altering properties. It has existed on Earth for milennia, and been refined down the ages by a few select groups. In ancient times, of course, they beleived it was magic. Some knew it as the "fountain of youth", "elixir of life" or "philosopher's stone". The latter came to the ear of Alcaemists, who beleived it was also the key to transmuting lead into gold. It isn't, but can be an ingredient of the "actual" philsopher's stone, which does.

If the right amount of the serum is introduced into somebody's bloodstream, it begins to infect cells and rewrite their DNA, partly so it can make more of itself, but also partly to render them biologically immortal - that is, able to always perfectly replicate and replace themselves. It does this to all body cells, even those which do not normally heal themselves properly, such as nerve cells. As the DNA information tells it what should go where, it also heals any wounds, even old scarring. Once a cell has become immortal, it stops producing the serum, so an infection of the serum eventually burns itself out, after passing through the whole body. The process is agonising, many people who take the serum often commit suicide before it is finished with them. Those who are forewarned of the effects tie themselves down first.

The immortality rendered by the serum is biological - a person can perfectly heal any wound, but this still takes time, rest, and enough food and water. If any major organs are damaged or destroyed, or a person is blown apart by a bomb, loses too much blood or is drowned or stopped from breathing, they will still die. They aren't invincible! That said, they can heal from very severe injuries, which few normal people survive, but this takes a long time, and they need to be given medical attention right away.

A variant of the immortality serum is produced by a true virus, this virus makes vampires. The bone marrow of vampires slowly produces more immortality serum, and they can choose to inject this into people to also convert them into vampires. As it is "carried" by the vampire virus, the infected person will always become a vampire, with all the nastiness that entails, and not a "normal" immortal.