Exposure to the hiv virus doesn't necessarily mean that a person will develop aids. some people have genetic resistance to infection by hiv. dr. stephen o'brien from the u. s. national cancer institute has recently identified a mutant form of a gene, called ccr5, that can protect against hiv infection. the mutation probably originated in europe among survivors of the bubonic plague. the mutated gene prevents the plague bacteria from attaching to cell membranes and, therefore, from entering and infecting body cells. although the hiv virus is very different from the bacteria that causes the plague, both diseases affect the exact same cells and use the same method of infection. the presence of the mutated gene in descendants of plague survivors prevent them from contracting aids. pharmaceutical companies are using this information as the basis for a new approach to aids prevention. this could be very important in areas of the world where the mutation is scarce or absent, such as africa. the most likely method by which the mutated ccr5 gene prevents aids is by