Where does Cootie think Henrietta's cancer came from?

From the text:

"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." 

In Cootie's eyes, the talk of Henrietta's cancer being "immortal" immediately linked it with voodoo.