1) leaven
 2) metaph. of inveterate mental and moral corruption, viewed in its
    tendency to infect others

 Leaven is applied to that which, though small in quantity, yet by its
 influence thoroughly pervades a thing; either in a good sense as in
 the parable \\#Mt 13:33\\; or in a bad sense, of a pernicious influence,
 "a little leaven leaveneth the whole lump"