In February 1951, Henrietta Lacks had a cell sample taken from a particularly nasty tumor. She died later that year, but in the hands of scientists, the sample lived much, much longer.
Even after the tumor was removed from Lacks' body, researchers found, it was ravenous. It devoured nutrients. If introduced into a dish with another type of cell, it would overwhelm it within weeks. And it multiplied and multiplied, in only two-thirds of the time it took other cells to do the same.
At which point it joined the Uncanny X-Men.
Lacks' cells spread like the plague among the scientific community, partly because of their incredible research value and also because they literally spread like a plague. They couldn't be contained -- they sneaked onto unrelated samples and traveled from lab to lab, some of the cells even making their way into Russia at the height of the Cold War.
HeLa, as the tumor was called, both to protect Lacks' identity and screw her family out of untold amounts of cash, is what's called an "immortal" cell line, meaning that it can reproduce infinitely, and it's also a laboratory "weed" that inevitably spreads and contaminates other cell samples. It has been a boon to the medical community and invaluable for research.
HeLa cells, seen here plotting world domination.