I saw an interesting show on the black plague on Discovery Health and they said that they found a mutation of the gene RRC5 and called it RRC5 Delta 32. It made people immune to AIDS if they have two copies of it and resistent to it if they have one copy. They also found higher percentages of that mutation in towns in England where the people are descendents of survivors of the plague. They tried putting plague on cultured cells and those with the mutation survived. It's because the mutation makes the gene defective and not make the protein on the cell surfaces that things like HIV use to get into the cells.
Stuff on the Mutation
Stuff on the Mutation