Question:

If leaven speaks of immorality, does defilement mean ceremonial or practical uncleanness?

Answer:

Leaven is spoken of in 1 Corinthians 5 (moral evil) and Galatians 5:9 (doctrinal evil).  A little leaven—one act of sin—spreads and affects the entire assembly.  The entire Corinthian assembly was leavened by having fellowship with the man in 1 Corinthians 5.  By ignoring the man’s sin, they were approving of his actions.  The sin doesn’t have to be ours personally, but it leavens us all.  We are affected by the sins of others, even if we don’t realize it.  This applies only to Christians living in sin.  When someone is put out (disciplined), God can work with that person.  The intent is to eventually restore the person.