Inoculation

     Cotton Mather, the character in this poem, was a real-life character who was a Puritan minister in Boston around Circa. 1700. He has been made famous by his heritage and also his involvement in the Salem Witch Trials. As dictated in the poem, Mather was a slave owner who had been studying Smallpox in Boston. The poem says, "...instead of sin," because of the fact that he was a renowned minister. So in his studies, he asks his slave, Onesimus, had he ever had the pox? Onesimus replied with yes and no because he never actually had the disease, but I believe he was referring to the disease of slavery, as he seems to emphasize it the second time he is questioned by Mather:

"'My mother bore me in the southern wild. 
She scratched my skin and I got sick, but lived
 to come here, free of  smallpox, as your slave.'"

     I think Onesimus is definitely suggesting that he overcame Smallpox, the initial disease, but he still has not overcome the subjugation of slavery, the latter "disease."