The religious leaders of Jesus' day considered themselves to be the proud custodians of a rich spiritual heritage. Of all the people of the earth God had called them - Abraham's descendants -to be the light of the world. But their exclusivism went even deeper. Within the chosen people there were people who had to be avoided because contact with them would cause righteous people to become contaminated. For good reason the Law prohibited contact with lepers and people with certain diseases, and by the time of Jesus they would have seen hidden lessons in the rules about which foods could be eaten and which could not ("clean" and "unclean" foods) about avoiding certain kinds of 'unclean' people: sinners, prostitutes, adulterers, tax collectors, homosexuals, etc.
They had a theory of contamination by contact which extended to guilt by association, so Jesus Himself, although he was "without sin" (Heb 4:15) was branded a sinner because He ate and socialised with 'sinners' (eg John 9:16).
A powerful example of how Jesus reacted to this contamination belief is in Luke 8 where we read about a woman who had had a disorder for 12 years which would have made her 'unclean'. Under the law she would have to avoid contact with other people. For example, she could not go into a busy street or market because she would contaminate anyone she touched. In a courageous act of faith this woman ventured out into a crowd in order to see Jesus. She tried to go unnoticed and came up behind Jesus and touched the edge of his cloak. Immediately she was healed of her disease. The most amazing thing about this story is not that she was healed by touching Jesus - awesome though that is - but in Jesus' reaction to it. "Who touched me?" He asked. His disciples clearly thought it was a ridiculoius question because they pointed out that He was in a crowd with people pressing and crushing against Him and each other. But Jesus said "someone touched me; I know that power has gone out from me" (v. 46).
This event is more than another miracle of many. Through it Jesus challenges an entire worldview. Under the Jewish law this woman would have contaminated anyone she touched, including Jesus. Defilement flowed from the unclean to the clean. But Jesus turned this around when He said power flowed from Him - from the clean to the unclean.
So in Jesus' worldview holiness - not sin - is contagious. He went beyond this and removed the authority of the laws of clean and unclean foods which had become the basis for the guilt-by-association theory (Mark 7:19 - He declared all foods clean). Later, after His ascension, He appeared to Peter in a dream and told him to eat animals which were 'unclean' and Peter thereafter preached the Gospel to a Gentile whom he would previously have considered 'unclean' and used this as the justification for eating and associating with Gentiles (Acts 11:1-18).
For many Christadelphians the guilt-by-association theory is as powerful as it was for the Pharisees of Jesus' day and they have failed to understand Jesus' teachings about this. Their belief prevents them from associating with non-believers, but worse still it is used as the justification for disfellowshipping people who disagree with them on doctrinal details, fail to uphold their 'standards', or who mix with Christadelphians in 'other fellowships'. The theory goes that if they don't expel these people that they too will be contaminated by their 'sin'. If you don't cut out the cancer the disease will spread. However what Jesus taught was the opposite! Holiness spreads, power goes out from Him to all He touches, and from them to all they touch with the Gospel. Paul said that unbelievers could even be sanctified by their believing partners (1 Corinthians 7:14) and uses Jesus' teaching on uncleanness to make his point: "Otherwise your children would be unclean, but as it is, they are holy".