How we cite our quotes:
'Tis but a surfeit, sir; fear nothing.
A surfeit of deadly sin that hath damned both body and soul. (5.2.36-38)
When Faustus complains that he's sick, the Scholars, drawing upon their medical knowledge, conclude that Faustus probably has an excess of something in his body. Back then, they thought that an excess of something like blood or bile was the root cause of a disease. Faustus turns their idea on its head, though, by acknowledging that he possesses an excess—of deadly sin, that is.