Numbers 5:1-6. God tells Moses to tell the people to put out from the camp any leper, anyone who has a discharge, and anyone who is unclean through having had contact with the dead. All of these, whether male or female, are to be driven out so that they do not defile the camp where God dwells. And the people obeyed and drove all of these people from the camp.

As usual, there is both a wordly and a spiritual interpretation of this. The people being driven out are contaminated and could contaminate others so that sickness and death would spread through the camp. Thus, even though it seems harsh and "un-Christian", the situation the people are in - living in tents and traveling through the desert - makes it such that the sick had to be driven out for the sake of the survival of the healthy.

But there is also a spiritual interpretation, as they're often is. Leprosy and contact with the dead can be seen as metaphors for sin. People who have allowed themselves to be in contact with great sin (death), or who are in a very sinful state (leprosy), will most likely begin to lead others astray. This happens all the time - one or two sinful people can bring down a whole group. So for the sake of his people, in the precarious time of desert traveling, God tells Moses that the people must drive the unclean from the camp in order to save the rest.

Sickness and sin are very similar things - sickness damages the physical body, while sin damages the soul. We should avoid prolonged contact with people who are suffering from either.