One of the great medical advances of the 19thC was the realisation that many diseases were caused by 'germs', which could be transmitted to spread the infection. Long before the discovery of antibiotics, disease was controlled by: thorough cleaning, sterilisation of surgical instruments, extra ventilation of wards and isolation of patients. But the principle of infection was known millennia before.
 
At the first Passover, just before God's people left Egypt, He instructed them to make unleavened bread (without any yeast 'germs') so that it would not go mouldy on the journey. The Passover celebrations after that, a memorial meal to thank God for delivering their ancestors from slavery, only used unleavened bread. To this day Jewish households will have a ritual cleaning of the whole house for a week to ensure that no yeast is in the house, which might contaminate the bread (Exodus 12:17-20).
 
Paul, like Jesus, likened the church's tolerance of sin to leaving yeast lying around in the house (Matthew 16:6). A little gust of wind would contaminate the cooking and spoil the whole batch. The Passover Lamb is Jesus Christ. His body is our 'bread of life' (John 6:35). How can we contaminate His purity by tolerating wickedness of any kind and the evil that propels it? Instead, Paul told them to ensure that the unrepentant sinner (whose incest was wrongly being celebrated by the church leaders) should be excluded from the church fellowship - lest his behaviour should infect others (1 Corinthians 5:1-5).
 
Sin contaminates. Our old nature will accept any kind of permission to feel that sin is acceptable. That is why leaders should not accept the unacceptable within the congregation. There is a remedy for sin, in the blood of Christ, but first there must be repentance. Without repentance, the church is at risk of greater contamination and judgement (Revelation 2:20-23). If it is still necessary to amputate an infected limb in order to save the whole body, why should modern churches think that tolerated sin is not a huge danger? Let us deal with whatever makes us unclean, and encourage others to do the same: then we can sincerely celebrate our relationship with Jesus (Galatians 6:1).