Leviticus 22:4-9

The priesthood received a portion of some of the offerings, some of the devoted things that Israel brought to be offered to God; but they were only allowed to eat it if they themselves were clean.

I think that for our purposes, it is important for us to understand that the defiling thing (that which makes a person unclean) may not, of itself, be sin—or that the person has necessarily sinned by coming into contact with it. It is what they represent, or symbolize, that is important to us here. We are dealing here with a ritual. The defiling thing represents, it symbolizes, it typifies sin and its effects. This is not to say, though, that coming into contact with some of these things might not be potentially physically harmful—because disease may very well be communicated by coming into contact with a corpse, for example. Nonetheless, the possibility of defilement by sin is taught in every case in which God declares a person unclean and therefore unsuited to serve Him until the defilement is taken away through the washing ceremonies prescribed by Him.