Everything about "pox leber" as a minced oath, and an earlier source.

[Bernd]

 "poxleber" might mean somebody who is sick (living with the pox)

No, German does not work like that. Also like Koen said, the dominant term for smallpox as well as other unrelated diseases was blattern / plattern.
I see no reason to assume pox- is not goat-related, especially as we have the drawing as well. 
I would rate a minced oath (potz-) far less likely but still magnitudes more likely than anything disease-related.