Evolutionary changes occur in populations of the viruses within an individual victim. In a typical victim, for example, a very small amount of AZT (azidothymidine) is all that is necessary to inactivate a large proportion of the viruses during early infection. By the second year of infection, much larger doses are needed to achieve the same effect. This occurs because the percentage of viruses that can resist AZT increase in the population of viruses. The figure on page 13 relates dosage of AZT to effectiveness; the horizontal axis is in powers of 10, which means that almost 10,000 times as much AZT was needed to kill about half the viruses in the second year of infection as in the second month in this particular person.

The most likely reason for the evolution of AZT-resistant viruses within an individual is that random mutations in the viral genes resulted in reverse transcriptase molecules that would not recognize AZT as a nucleotide. While a mutant reverse transcriptase molecule would normally be detrimental to a virus, in the presence of AZT this mutant enzyme, though somewhat defective, is at least able to operate. Therefore, mutant viruses thrive in the presence, but not in the absence, of AZT. This pattern, in which resistant organisms thrive in the presence of the chemical agent used against them but are otherwise inferior to the susceptible organisms, is general among the many cases of the evolution of resistance to antibiotics, pesticides, and herbicides.

Resistance is less likely to evolve if several different chemical agents are used together. This is the reason that many different antibiotics, pesticides, and herbicides are in use and more are being developed. Populations of HIV can evolve resistance to any of the chemical treatments against it (reverse transcriptase inhibitors such as AZT; chemicals that inhibit proteases; chemicals that block the entry of HIV into white blood cells; chemicals that block the integration of viral DNA into host chromosomes) but a combination or "cocktail" of different chemicals has proven effective at stopping the spread of HIV within a victim. It is much less likely that any virus will happen to possess mutations that render it resistant to all four means of chemical control than that it will possess a mutation against any one of them. In fact, mathematical calculations show that a cocktail of three chemicals is much more than three times as effective as each chemical individually. . .

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