The answer to this question can be found in Chapter 10 of this text. In this chapter, the author refers to the animistic religious beliefs of the Lacks family and the way that they feel harm and problems are visited upon somebody not by chance but by the bad will of other people. Note what Cootie says to the author about his own personal theory of how Henrietta got her cancer:
"You know, a lot of things, they man-made," he told me, dropping his voice to a whisper. "You know what I mean by man-made, don't you?... Voodoo... Some peoples is sayin Henrietta's sickness and them cells was man- or woman-made, others say it was doctor-made."
Cootie therefore repeats a rumour that Henrietta's cancer, precisely because it has carried on growing and shown itself to be immortal, could not have been a "normal" disease, and that it therefore must have been the result of some supernatural origin, which Cootie ascribes to voodoo. The very fact that there have been no other cases of cells that just keep on growing indefinitely proves this point without a doubt to Cootie: Henrietta's cancer was not natural.