From where did the term night doctors originate and why is the name meaningful?

From the text:

Since at least the 1800s, black oral history has been filled with tales of “night doctors” who kidnapped black people for research … To discourage slaves from meeting or escaping, slave owners told tales of gruesome research done on black bodies, then covered themselves in white sheets and crept around at night, posing as spirits coming to infect black people with disease or steal them for research.

The term is especially meaningful in relation to Henrietta Lacks, as her family believed the doctors had done just this in using her cells without permission. 

The Immortal Life of Henrietta Lacks, pg. 165-166