I think sin can be portrayed as a disease, most accurately as leprosy. Isaiah 64:6 "But we are all as an unclean thing, and all our righteousnesses are as filthy rags...". Unclean, in this context, is referring to a leprous person, who were required to declare their status "Unclean!" in public. And the "filthy rags" referring to the rags used to wrap the leprous spots, which would catch the oozing from the wounds. (Gross, right?). Further, Leprosy was a disease that began inward, and unnoticed. In the end, it was very much noticed by everyone. It was also impossible to be cured by yourself, and required supernatural intervention. Sounds like a pretty good picture of sin. Sin [singular] is an inward issue of the heart, which manifests itself as sins [pl.] on the outside.
But personally, I take my view of sin from the first mention of it in Scripture, Genesis 4:7 " If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him."
The verb for "lieth" in this passage is attributive of an animal lying in wait. Sin is personified as an animal, "lying in wait" at your doorstep, waiting for you to "not do well". This "animal" Sin is further personified by saying he has a desire, and he is to be ruled over. Now, I'll use the Hebrew word used here for sin, "Khata", to refer to this "animal".
It's interesting, because "if thou doest not well" has the same meaning as "if you sin" (Sin is "missing the mark", failure to "do well" and be accepted, and "falling short"). But, in this passage, it's basically saying (paraphrase) "If you sin, then Khata lieth in wait at the door for you...". Also, take note it says "sin lieth at the door" and not "your sins...". Sin is understood as a singular being, and not necessarily just a list of our particular transgressions.
So, if you chose the path of sin, then Khata is there waiting for you. He will seize the opportunity to pounce on you, and have his way with you, like a lion plays with it's defenseless victim. As the scripture says, "Unto thee shall be his desire". How sad, that we would rather play with this animal as if it were a puppy!
So, I think sin is portrayed as both a leprous disease, and also a ravening animal. Both carry different images with it.