I was just studying into this and noticed that Jesus talked about two kinds of leaven, the leaven of the Pharisees, and the leaven of Herod (Mark 8:15). Jesus frequently teaches in parables and illustrations and here, speaking of leaven, he refers to an infectious spiritual disease that anyone could catch if they were not careful.
Sometimes we find ourselves walking around sick and not aware of exactly what that sickness is. So it is with the sickness of the Pharisees and of Herod. If people have been unusually avoiding you perhaps you should get a check-up.
The Pharisee Sickness
Symptoms would include hypocrisy (Mark 12:13-15), legalism (Matt. 23:23-24), self-righteousness (Matt. 23:28), haughtiness (Mark 15:31), spiritual pride (Luke 18:11-12), and judgmentalism (Matt. 23:34). This illness is more internal and less outwardly manifest and as such, tends to cause you to hide behind a pretty face while living in a lie.
The Herod Sickness
Symptoms would include, immorality, lasciviousness, indifference, apathy, mockery, debauchery, drunkenness, filthy speech, and the like. This illness shows itself plainly, and patients usually exhibit a defensive attitude towards their debauched way of living. (Matt. 2:16; Mark 7:21-23; Eph. 5:18; Eph. 5:4; 1 Peter 4:3; Gal. 5:19-24)
It is best to avoid these kind of people (2 Timothy 3:1-5). Of course we mean those who call themselves brothers and are sick like this (1 Cor. 5:9-13). If you are around them too much you risk the chance of becoming infected (1 Cor. 15:33, Luke 21:34).
If you are infected with such a disease yourself, take heart, Jesus can heal you (Matt. 12:15; Rev. 3:17-20)!